The following commit has been merged into the ras/core branch of tip: Commit-ID: 3376136300a00df9a864b88fa969177d6c3be8e5 Gitweb: https://git.kernel.org/tip/3376136300a00df9a864b88fa969177d6c3be8e5 Author: Youquan Song <youquan.song@xxxxxxxxx> AuthorDate: Thu, 23 Dec 2021 12:07:01 -08:00 Committer: Borislav Petkov <bp@xxxxxxx> CommitterDate: Fri, 31 Dec 2021 18:22:32 +01:00 x86/mce: Reduce number of machine checks taken during recovery When any of the copy functions in arch/x86/lib/copy_user_64.S take a fault, the fixup code copies the remaining byte count from %ecx to %edx and unconditionally jumps to .Lcopy_user_handle_tail to continue the copy in case any more bytes can be copied. If the fault was #PF this may copy more bytes (because the page fault handler might have fixed the fault). But when the fault is a machine check the original copy code will have copied all the way to the poisoned cache line. So .Lcopy_user_handle_tail will just take another machine check for no good reason. Every code path to .Lcopy_user_handle_tail comes from an exception fixup path, so add a check there to check the trap type (in %eax) and simply return the count of remaining bytes if the trap was a machine check. Doing this reduces the number of machine checks taken during synthetic tests from four to three. As well as reducing the number of machine checks, this also allows Skylake generation Xeons to recover some cases that currently fail. The is because REP; MOVSB is only recoverable when source and destination are well aligned and the byte count is large. That useless call to .Lcopy_user_handle_tail may violate one or more of these conditions and generate a fatal machine check. [ Tony: Add more details to commit message. ] [ bp: Fixup comment. Also, another tip patchset which is adding straight-line speculation mitigation changes the "ret" instruction to an all-caps macro "RET". But, since gas is case-insensitive, use "RET" in the newly added asm block already in order to simplify tip branch merging on its way upstream. ] Signed-off-by: Youquan Song <youquan.song@xxxxxxxxx> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx> Signed-off-by: Borislav Petkov <bp@xxxxxxx> Link: https://lore.kernel.org/r/YcTW5dh8yTGucDd+@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx --- arch/x86/lib/copy_user_64.S | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S index 2797e63..e73b76e 100644 --- a/arch/x86/lib/copy_user_64.S +++ b/arch/x86/lib/copy_user_64.S @@ -225,6 +225,7 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string) * Don't try to copy the tail if machine check happened * * Input: + * eax trap number written by ex_handler_copy() * rdi destination * rsi source * rdx count @@ -233,12 +234,20 @@ EXPORT_SYMBOL(copy_user_enhanced_fast_string) * eax uncopied bytes or 0 if successful. */ SYM_CODE_START_LOCAL(.Lcopy_user_handle_tail) + cmp $X86_TRAP_MC,%eax + je 3f + movl %edx,%ecx 1: rep movsb 2: mov %ecx,%eax ASM_CLAC ret +3: + movl %edx,%eax + ASM_CLAC + RET + _ASM_EXTABLE_CPY(1b, 2b) SYM_CODE_END(.Lcopy_user_handle_tail)
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