Messages in this thread | | | From | Linus Torvalds <> | Date | Tue, 6 Feb 2018 12:26:32 -0800 | Subject | Re: [PATCH v4 07/10] x86: narrow out of bounds syscalls to sys_read under speculation |
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On Tue, Feb 6, 2018 at 11:48 AM, Dan Williams <dan.j.williams@intel.com> wrote: > > Just to clarify, when you say "this patch" you mean: > > 2fbd7af5af86 x86/syscall: Sanitize syscall table de-references > under speculation > > ...not this early MASK_NOSPEC version of the patch, right?
I suspect not. If that patch is broken, the system wouldn't even boot.
That said, looking at 2fbd7af5af86, I do note that the code generation is horribly stupid.
It's due to two different issues:
(a) the x86 asm constraints for that inline asm is nasty, and requires a register for 'size', even though an immediate works just fine.
(b) the "cmp" is inside the asm, so gcc can't combine it with the *other* cmp in the C code.
Fixing (a) is easy:
+++ b/arch/x86/include/asm/barrier.h @@ -43 +43 @@ static inline unsigned long array_index_mask_nospec(unsigned long index, - :"r"(size),"r" (index) + :"ir"(size),"r" (index)
but fixing (b) looks fundamentally hard. Gcc generates (for do_syscall()):
cmpq $332, %rbp #, nr ja .L295 #, cmp $333,%rbp sbb %rax,%rax; #, nr, mask
note how it completely pointlessly does the comparison twice, even though it could have just done
cmp $333,%rbp jae .L295 #, sbb %rax,%rax; #, nr, mask
Ho humm. Sad.
Linus
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