70 lines
2.7 KiB
Text
70 lines
2.7 KiB
Text
# test_zeroize.gdb
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#
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# This file is part of Mbed TLS (https://tls.mbed.org)
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#
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# Copyright (c) 2018, Arm Limited, All Rights Reserved
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#
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# Purpose
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#
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# Run a test using the debugger to check that the mbedtls_platform_zeroize()
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# function in platform_util.h is not being optimized out by the compiler. To do
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# so, the script loads the test program at programs/test/zeroize.c and sets a
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# breakpoint at the last return statement in main(). When the breakpoint is
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# hit, the debugger manually checks the contents to be zeroized and checks that
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# it is actually cleared.
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#
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# The mbedtls_platform_zeroize() test is debugger driven because there does not
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# seem to be a mechanism to reliably check whether the zeroize calls are being
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# eliminated by compiler optimizations from within the compiled program. The
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# problem is that a compiler would typically remove what it considers to be
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# "unecessary" assignments as part of redundant code elimination. To identify
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# such code, the compilar will create some form dependency graph between
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# reads and writes to variables (among other situations). It will then use this
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# data structure to remove redundant code that does not have an impact on the
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# program's observable behavior. In the case of mbedtls_platform_zeroize(), an
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# intelligent compiler could determine that this function clears a block of
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# memory that is not accessed later in the program, so removing the call to
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# mbedtls_platform_zeroize() does not have an observable behavior. However,
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# inserting a test after a call to mbedtls_platform_zeroize() to check whether
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# the block of memory was correctly zeroed would force the compiler to not
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# eliminate the mbedtls_platform_zeroize() call. If this does not occur, then
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# the compiler potentially has a bug.
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#
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# Note: This test requires that the test program is compiled with -g3.
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#
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# WARNING: There does not seem to be a mechanism in GDB scripts to set a
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# breakpoint at the end of a function (probably because there are a lot of
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# complications as function can have multiple exit points, etc). Therefore, it
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# was necessary to hard-code the line number of the breakpoint in the zeroize.c
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# test app. The assumption is that zeroize.c is a simple test app that does not
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# change often (as opposed to the actual library code), so the breakpoint line
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# number does not need to be updated often.
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set confirm off
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file ./programs/test/zeroize
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break zeroize.c:100
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set args ./programs/test/zeroize.c
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run
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set $i = 0
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set $len = sizeof(buf)
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set $buf = buf
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while $i < $len
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if $buf[$i++] != 0
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echo The buffer at was not zeroized\n
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quit 1
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end
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end
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echo The buffer was correctly zeroized\n
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continue
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if $_exitcode != 0
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echo The program did not terminate correctly\n
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quit 1
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end
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quit 0
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