196 lines
8.5 KiB
ReStructuredText
196 lines
8.5 KiB
ReStructuredText
DataFlowSanitizer Design Document
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=================================
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This document sets out the design for DataFlowSanitizer, a general
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dynamic data flow analysis. Unlike other Sanitizer tools, this tool is
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not designed to detect a specific class of bugs on its own. Instead,
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it provides a generic dynamic data flow analysis framework to be used
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by clients to help detect application-specific issues within their
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own code.
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DataFlowSanitizer is a program instrumentation which can associate
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a number of taint labels with any data stored in any memory region
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accessible by the program. The analysis is dynamic, which means that
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it operates on a running program, and tracks how the labels propagate
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through that program.
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Use Cases
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---------
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This instrumentation can be used as a tool to help monitor how data
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flows from a program's inputs (sources) to its outputs (sinks).
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This has applications from a privacy/security perspective in that
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one can audit how a sensitive data item is used within a program and
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ensure it isn't exiting the program anywhere it shouldn't be.
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Interface
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---------
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A number of functions are provided which will attach taint labels to
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memory regions and extract the set of labels associated with a
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specific memory region. These functions are declared in the header
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file ``sanitizer/dfsan_interface.h``.
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.. code-block:: c
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/// Sets the label for each address in [addr,addr+size) to \c label.
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void dfsan_set_label(dfsan_label label, void *addr, size_t size);
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/// Sets the label for each address in [addr,addr+size) to the union of the
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/// current label for that address and \c label.
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void dfsan_add_label(dfsan_label label, void *addr, size_t size);
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/// Retrieves the label associated with the given data.
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///
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/// The type of 'data' is arbitrary. The function accepts a value of any type,
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/// which can be truncated or extended (implicitly or explicitly) as necessary.
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/// The truncation/extension operations will preserve the label of the original
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/// value.
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dfsan_label dfsan_get_label(long data);
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/// Returns whether the given label label contains the label elem.
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int dfsan_has_label(dfsan_label label, dfsan_label elem);
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/// Computes the union of \c l1 and \c l2, resulting in a union label.
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dfsan_label dfsan_union(dfsan_label l1, dfsan_label l2);
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Taint label representation
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--------------------------
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We use an 8-bit unsigned integer for the representation of a
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label. The label identifier 0 is special, and means that the data item
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is unlabelled. This is optimizing for low CPU and code size overhead
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of the instrumentation. When a label union operation is requested at a
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join point (any arithmetic or logical operation with two or more
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operands, such as addition), we can simply OR the two labels in O(1).
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Users are responsible for managing the 8 integer labels (i.e., keeping
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track of what labels they have used so far, picking one that is yet
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unused, etc).
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Memory layout and label management
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----------------------------------
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The following is the memory layout for Linux/x86\_64:
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+---------------+---------------+--------------------+
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| Start | End | Use |
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+===============+===============+====================+
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| 0x700000000000|0x800000000000 | application 3 |
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+---------------+---------------+--------------------+
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| 0x610000000000|0x700000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x600000000000|0x610000000000 | origin 1 |
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+---------------+---------------+--------------------+
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| 0x510000000000|0x600000000000 | application 2 |
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+---------------+---------------+--------------------+
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| 0x500000000000|0x510000000000 | shadow 1 |
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+---------------+---------------+--------------------+
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| 0x400000000000|0x500000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x300000000000|0x400000000000 | origin 3 |
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+---------------+---------------+--------------------+
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| 0x200000000000|0x300000000000 | shadow 3 |
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+---------------+---------------+--------------------+
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| 0x110000000000|0x200000000000 | origin 2 |
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+---------------+---------------+--------------------+
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| 0x100000000000|0x110000000000 | unused |
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+---------------+---------------+--------------------+
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| 0x010000000000|0x100000000000 | shadow 2 |
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+---------------+---------------+--------------------+
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| 0x000000000000|0x010000000000 | application 1 |
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+---------------+---------------+--------------------+
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Each byte of application memory corresponds to a single byte of shadow
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memory, which is used to store its taint label. We map memory, shadow, and
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origin regions to each other with these masks and offsets:
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* shadow_addr = memory_addr ^ 0x500000000000
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* origin_addr = shadow_addr + 0x100000000000
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As for LLVM SSA registers, we have not found it necessary to associate a label
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with each byte or bit of data, as some other tools do. Instead, labels are
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associated directly with registers. Loads will result in a union of
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all shadow labels corresponding to bytes loaded, and stores will
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result in a copy of the label of the stored value to the shadow of all
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bytes stored to.
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Propagating labels through arguments
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------------------------------------
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In order to propagate labels through function arguments and return values,
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DataFlowSanitizer changes the ABI of each function in the translation unit.
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There are currently two supported ABIs:
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* Args -- Argument and return value labels are passed through additional
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arguments and by modifying the return type.
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* TLS -- Argument and return value labels are passed through TLS variables
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``__dfsan_arg_tls`` and ``__dfsan_retval_tls``.
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The main advantage of the TLS ABI is that it is more tolerant of ABI mismatches
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(TLS storage is not shared with any other form of storage, whereas extra
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arguments may be stored in registers which under the native ABI are not used
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for parameter passing and thus could contain arbitrary values). On the other
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hand the args ABI is more efficient and allows ABI mismatches to be more easily
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identified by checking for nonzero labels in nominally unlabelled programs.
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Implementing the ABI list
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-------------------------
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The `ABI list <DataFlowSanitizer.html#abi-list>`_ provides a list of functions
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which conform to the native ABI, each of which is callable from an instrumented
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program. This is implemented by replacing each reference to a native ABI
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function with a reference to a function which uses the instrumented ABI.
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Such functions are automatically-generated wrappers for the native functions.
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For example, given the ABI list example provided in the user manual, the
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following wrappers will be generated under the args ABI:
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.. code-block:: llvm
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define linkonce_odr { i8*, i16 } @"dfsw$malloc"(i64 %0, i16 %1) {
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entry:
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%2 = call i8* @malloc(i64 %0)
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%3 = insertvalue { i8*, i16 } undef, i8* %2, 0
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%4 = insertvalue { i8*, i16 } %3, i16 0, 1
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ret { i8*, i16 } %4
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}
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define linkonce_odr { i32, i16 } @"dfsw$tolower"(i32 %0, i16 %1) {
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entry:
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%2 = call i32 @tolower(i32 %0)
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%3 = insertvalue { i32, i16 } undef, i32 %2, 0
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%4 = insertvalue { i32, i16 } %3, i16 %1, 1
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ret { i32, i16 } %4
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}
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define linkonce_odr { i8*, i16 } @"dfsw$memcpy"(i8* %0, i8* %1, i64 %2, i16 %3, i16 %4, i16 %5) {
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entry:
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%labelreturn = alloca i16
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%6 = call i8* @__dfsw_memcpy(i8* %0, i8* %1, i64 %2, i16 %3, i16 %4, i16 %5, i16* %labelreturn)
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%7 = load i16* %labelreturn
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%8 = insertvalue { i8*, i16 } undef, i8* %6, 0
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%9 = insertvalue { i8*, i16 } %8, i16 %7, 1
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ret { i8*, i16 } %9
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}
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As an optimization, direct calls to native ABI functions will call the
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native ABI function directly and the pass will compute the appropriate label
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internally. This has the advantage of reducing the number of union operations
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required when the return value label is known to be zero (i.e. ``discard``
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functions, or ``functional`` functions with known unlabelled arguments).
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Checking ABI Consistency
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------------------------
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DFSan changes the ABI of each function in the module. This makes it possible
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for a function with the native ABI to be called with the instrumented ABI,
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or vice versa, thus possibly invoking undefined behavior. A simple way
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of statically detecting instances of this problem is to append the suffix
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".dfsan" to the name of each instrumented-ABI function.
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This will not catch every such problem; in particular function pointers passed
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across the instrumented-native barrier cannot be used on the other side.
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These problems could potentially be caught dynamically.
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