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Kees Cook 0e108725f6 string: Adjust strtomem() logic to allow for smaller sources
Arnd noticed we have a case where a shorter source string is being copied
into a destination byte array, but this results in a strnlen() call that
exceeds the size of the source. This is seen with -Wstringop-overread:

In file included from ../include/linux/uuid.h:11,
                 from ../include/linux/mod_devicetable.h:14,
                 from ../include/linux/cpufeature.h:12,
                 from ../arch/x86/coco/tdx/tdx.c:7:
../arch/x86/coco/tdx/tdx.c: In function 'tdx_panic.constprop':
../include/linux/string.h:284:9: error: 'strnlen' specified bound 64 exceeds source size 60 [-Werror=stringop-overread]
  284 |         memcpy_and_pad(dest, _dest_len, src, strnlen(src, _dest_len), pad); \
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../arch/x86/coco/tdx/tdx.c:124:9: note: in expansion of macro 'strtomem_pad'
  124 |         strtomem_pad(message.str, msg, '\0');
      |         ^~~~~~~~~~~~

Use the smaller of the two buffer sizes when calling strnlen(). When
src length is unknown (SIZE_MAX), it is adjusted to use dest length,
which is what the original code did.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: dfbafa70bd ("string: Introduce strtomem() and strtomem_pad()")
Tested-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
History 2023-10-18 17:56:32 -07:00
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Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.