From 4e26d11f52684dc8b1632a8cfe450cb5197a8464 Mon Sep 17 00:00:00 2001 From: Hector Marco-Gisbert Date: Fri, 27 Mar 2015 12:38:21 +0100 Subject: [PATCH] x86/mm: Improve AMD Bulldozer ASLR workaround 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: dfb09f9b7ab0 ("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.pdf Mentored-by: Ismael Ripoll Signed-off-by: Hector Marco-Gisbert Signed-off-by: Borislav Petkov Acked-by: Kees Cook Cc: Alexander Viro Cc: Andrew Morton Cc: H. Peter Anvin Cc: Jan-Simon Cc: Thomas Gleixner Cc: linux-fsdevel@vger.kernel.org Link: http://lkml.kernel.org/r/1427456301-3764-1-git-send-email-hecmargi@upv.es Signed-off-by: Ingo Molnar --- arch/x86/include/asm/elf.h | 1 + arch/x86/kernel/cpu/amd.c | 4 ++++ arch/x86/kernel/sys_x86_64.c | 30 +++++++++++++++++++++++++++--- 3 files changed, 32 insertions(+), 3 deletions(-) diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h index ca3347a9dab5..bd292ce9be0a 100644 --- a/arch/x86/include/asm/elf.h +++ b/arch/x86/include/asm/elf.h @@ -365,6 +365,7 @@ enum align_flags { struct va_alignment { int flags; unsigned long mask; + unsigned long bits; } ____cacheline_aligned; extern struct va_alignment va_align; diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c index a220239cea65..ec6a61b21b41 100644 --- a/arch/x86/kernel/cpu/amd.c +++ b/arch/x86/kernel/cpu/amd.c @@ -5,6 +5,7 @@ #include #include +#include #include #include #include @@ -488,6 +489,9 @@ static void bsp_init_amd(struct cpuinfo_x86 *c) va_align.mask = (upperbit - 1) & PAGE_MASK; va_align.flags = ALIGN_VA_32 | ALIGN_VA_64; + + /* A random value per boot for bit slice [12:upper_bit) */ + va_align.bits = get_random_int() & va_align.mask; } } diff --git a/arch/x86/kernel/sys_x86_64.c b/arch/x86/kernel/sys_x86_64.c index 30277e27431a..10e0272d789a 100644 --- a/arch/x86/kernel/sys_x86_64.c +++ b/arch/x86/kernel/sys_x86_64.c @@ -34,10 +34,26 @@ static unsigned long get_align_mask(void) return va_align.mask; } +/* + * To avoid aliasing in the I$ on AMD F15h, the bits defined by the + * va_align.bits, [12:upper_bit), are set to a random value instead of + * zeroing them. This random value is computed once per boot. This form + * of ASLR is known as "per-boot ASLR". + * + * To achieve this, the random value is added to the info.align_offset + * value before calling vm_unmapped_area() or ORed directly to the + * address. + */ +static unsigned long get_align_bits(void) +{ + return va_align.bits & get_align_mask(); +} + unsigned long align_vdso_addr(unsigned long addr) { unsigned long align_mask = get_align_mask(); - return (addr + align_mask) & ~align_mask; + addr = (addr + align_mask) & ~align_mask; + return addr | get_align_bits(); } static int __init control_va_addr_alignment(char *str) @@ -135,8 +151,12 @@ arch_get_unmapped_area(struct file *filp, unsigned long addr, info.length = len; info.low_limit = begin; info.high_limit = end; - info.align_mask = filp ? get_align_mask() : 0; + info.align_mask = 0; info.align_offset = pgoff << PAGE_SHIFT; + if (filp) { + info.align_mask = get_align_mask(); + info.align_offset += get_align_bits(); + } return vm_unmapped_area(&info); } @@ -174,8 +194,12 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0, info.length = len; info.low_limit = PAGE_SIZE; info.high_limit = mm->mmap_base; - info.align_mask = filp ? get_align_mask() : 0; + info.align_mask = 0; info.align_offset = pgoff << PAGE_SHIFT; + if (filp) { + info.align_mask = get_align_mask(); + info.align_offset += get_align_bits(); + } addr = vm_unmapped_area(&info); if (!(addr & ~PAGE_MASK)) return addr; -- GitLab