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由 Hector Marco-Gisbert 提交于
The ASLR implementation needs to special-case AMD F15h processors by clearing out bits [14:12] of the virtual address in order to avoid I$ cross invalidations and thus performance penalty for certain workloads. For details, see: dfb09f9b ("x86, amd: Avoid cache aliasing penalties on AMD family 15h") This special case reduces the mmapped file's entropy by 3 bits. The following output is the run on an AMD Opteron 62xx class CPU processor under x86_64 Linux 4.0.0: $ for i in `seq 1 10`; do cat /proc/self/maps | grep "r-xp.*libc" ; done b7588000-b7736000 r-xp 00000000 00:01 4924 /lib/i386-linux-gnu/libc.so.6 b7570000-b771e000 r-xp 00000000 00:01 4924 /lib/i386-linux-gnu/libc.so.6 b75d0000-b777e000 r-xp 00000000 00:01 4924 /lib/i386-linux-gnu/libc.so.6 b75b0000-b775e000 r-xp 00000000 00:01 4924 /lib/i386-linux-gnu/libc.so.6 b7578000-b7726000 r-xp 00000000 00:01 4924 /lib/i386-linux-gnu/libc.so.6 ... Bits [12:14] are always 0, i.e. the address always ends in 0x8000 or 0x0000. 32-bit systems, as in the example above, are especially sensitive to this issue because 32-bit randomness for VA space is 8 bits (see mmap_rnd()). With the Bulldozer special case, this diminishes to only 32 different slots of mmap virtual addresses. This patch randomizes per boot the three affected bits rather than setting them to zero. Since all the shared pages have the same value at bits [12..14], there is no cache aliasing problems. This value gets generated during system boot and it is thus not known to a potential remote attacker. Therefore, the impact from the Bulldozer workaround gets diminished and ASLR randomness increased. More details at: http://hmarco.org/bugs/AMD-Bulldozer-linux-ASLR-weakness-reducing-mmaped-files-by-eight.html Original white paper by AMD dealing with the issue: http://developer.amd.com/wordpress/media/2012/10/SharedL1InstructionCacheonAMD15hCPU.pdfMentored-by: NIsmael Ripoll <iripoll@disca.upv.es> Signed-off-by: NHector Marco-Gisbert <hecmargi@upv.es> Signed-off-by: NBorislav Petkov <bp@suse.de> Acked-by: NKees Cook <keescook@chromium.org> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Jan-Simon <dl9pf@gmx.de> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: linux-fsdevel@vger.kernel.org Link: http://lkml.kernel.org/r/1427456301-3764-1-git-send-email-hecmargi@upv.esSigned-off-by: NIngo Molnar <mingo@kernel.org>
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