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| author | Andy Lutomirski <luto@kernel.org> | 2015-04-01 21:26:34 (GMT) |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2015-04-02 09:09:54 (GMT) |
| commit | 7ea24169097d3d3a3eab2dcc5773bc43fd5593e7 (patch) | |
| tree | 5c8663df0bc9f134b4d97b283280784aa846ca3b | |
| parent | 80313b3078fcd2ca51970880d90757f05879a193 (diff) | |
| download | linux-7ea24169097d3d3a3eab2dcc5773bc43fd5593e7.tar.xz | |
x86/asm/entry/64: Disable opportunistic SYSRET if regs->flags has TF set
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"
"nop\n\t"
"post_nop:"
: : "c" (post_nop) : "r11");
In my defense, I can't find this documented in the AMD or Intel manual.
Fix it by using IRET to restore TF.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 2a23c6b8a9c4 ("x86_64, entry: Use sysret to return to userspace when possible")
Link: http://lkml.kernel.org/r/9472f1ca4c19a38ecda45bba9c91b7168135fcfa.1427923514.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
| -rw-r--r-- | arch/x86/kernel/entry_64.S | 16 |
1 files changed, 15 insertions, 1 deletions
diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 2babb39..f0095a7 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -799,7 +799,21 @@ retint_swapgs: /* return to user-space */ cmpq %r11,(EFLAGS-ARGOFFSET)(%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 */ |