Re: [PATCH] x86_64, entry: Fix out of bounds read on sysenter

[Rusty Russell]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> writes:
> Rusty noticed a Really Bad Bug (tm) in my NT fix.  The entry code
> reads out of bounds, causing the NT fix to be unreliable.  But, and
> this is much, much worse, if your stack is somehow just below the
> top of the direct map (or a hole), you read out of bounds and crash.
>
> Excerpt from the crash:
>
> [    1.129513] RSP: 0018:ffff88001da4bf88  EFLAGS: 00010296
>
>   2b:*    f7 84 24 90 00 00 00     testl  $0x4000,0x90(%rsp)
>
> That read is deterministically above the top of the stack.  I
> thought I even single-stepped through this code when I wrote it to
> check the offset, but I clearly screwed it up.
>
> Fixes 8c7aa698baca x86_64, entry: Filter RFLAGS.NT on entry from userspace
>
> Reported-by: Rusty Russell <rusty@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

Tested-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx>

Thanks for the fast response...
Rusty.
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