Re: [PATCH urgent v2] x86, asm: Disable opportunistic SYSRET if regs->flags has TF set
From: Borislav Petkov
Date: Thu Apr 02 2015 - 02:23:17 EST
On Wed, Apr 01, 2015 at 02:26:34PM -0700, Andy Lutomirski wrote:
> When I wrote the opportunistic SYSRET code, I missed an important
> difference between SYSRET and IRET. Both instructions are capable
> of setting EFLAGS.TF, but they behave differently when doing so.
> IRET will not issue a #DB trap after execution when it sets TF This
> is critical -- otherwise you'd never be able to make forward
> progress when returning to userspace. SYSRET, on the other hand,
> will trap with #DB immediately after returning to CPL3, and the next
> instruction will never execute.
> This breaks anything that opportunistically SYSRETs to a user
> context with TF set. For example, running this code with TF set and
> a SIGTRAP handler loaded never gets past post_nop.
> extern unsigned char post_nop;
> asm volatile ("pushfq\n\t"
> "popq %%r11\n\t"
> : : "c" (post_nop) : "r11");
> In my defense, I can't find this documented in the AMD or Intel
> Fix it by using IRET to restore TF.
> Fixes: 2a23c6b8a9c4 x86_64, entry: Use sysret to return to userspace when possible
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> This affects 4.0-rc as well as -tip. A full test case lives here:
> It's called single_step_syscall_64.
> On Intel systems, the 32-bit version of that test fails for unrelated
> reasons, but that's not a regression, and fixing it will be much more
> Changes from v1:
> - Remove mention of testl from changelog.
> - Improve comment per Denys' suggestion.
> arch/x86/kernel/entry_64.S | 16 +++++++++++++++-
> 1 file changed, 15 insertions(+), 1 deletion(-)
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index 750c6efcb718..537716380959 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -715,7 +715,21 @@ retint_swapgs: /* return to user-space */
> cmpq %r11,EFLAGS(%rsp) /* R11 == RFLAGS */
> jne opportunistic_sysret_failed
> - testq $X86_EFLAGS_RF,%r11 /* sysret can't restore RF */
> + /*
> + * SYSRET can't restore RF. SYSRET can restore TF, but unlike IRET,
> + * restoring TF results in a trap from userspace immediately after
> + * SYSRET. This would cause an infinite loop whenever #DB happens
> + * with register state that satisfies the opportunistic SYSRET
> + * conditions. For example, single-stepping this user code:
> + *
> + * movq $stuck_here,%rcx
> + * pushfq
> + * popq %r11
> + * stuck_here:
> + *
> + * would never get past stuck_here.
> + */
> + testq $(X86_EFLAGS_RF|X86_EFLAGS_TF),%r11
> jnz opportunistic_sysret_failed
> /* nothing to check for RSP */
Acked-by: Borislav Petkov <bp@xxxxxxx>
ECO tip #101: Trim your mails when you reply.
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