From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Barret Rhoden <brho@google.com>,
Josh Bleecher Snyder <josharian@gmail.com>
Cc: "Rik van Riel\"" <riel@surriel.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>
Subject: [PATCH] x86/fpu: Don't cache access to fpu_fpregs_owner_ctx
Date: Wed, 27 Nov 2019 13:42:43 +0100 [thread overview]
Message-ID: <20191127124243.u74osvlkhcmsskng@linutronix.de> (raw)
In-Reply-To: <e4d6406b-0d47-5cc5-f3a8-6d14bd90760b@google.com>
The state/owner of FPU is saved fpu_fpregs_owner_ctx by pointing to the
context that is currently loaded. It never changed during the life time
of a task and remained stable/constant.
Since we deferred loading the FPU registers on return to userland, the
content of fpu_fpregs_owner_ctx may change during preemption and must
not be cached.
This went unnoticed for some time and was now noticed, in particular
gcc-9 is able to cache that load in copy_fpstate_to_sigframe() and reuse
it in the retry loop:
copy_fpstate_to_sigframe()
load fpu_fpregs_owner_ctx and save on stack
fpregs_lock()
copy_fpregs_to_sigframe() /* failed */
fpregs_unlock()
*** PREEMPTION, another uses FPU, changes fpu_fpregs_owner_ctx ***
fault_in_pages_writeable() /* succeed, retry */
fpregs_lock()
__fpregs_load_activate()
fpregs_state_valid() /* uses fpu_fpregs_owner_ctx from stack */
copy_fpregs_to_sigframe() /* succeeds, random FPU content */
This is a comparison of the assembly of gcc-9, without vs with this
patch:
| # arch/x86/kernel/fpu/signal.c:173: if (!access_ok(buf, size))
| cmpq %rdx, %rax # tmp183, _4
| jb .L190 #,
|-# arch/x86/include/asm/fpu/internal.h:512: return fpu == this_cpu_read_stable(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
|-#APP
|-# 512 "arch/x86/include/asm/fpu/internal.h" 1
|- movq %gs:fpu_fpregs_owner_ctx,%rax #, pfo_ret__
|-# 0 "" 2
|-#NO_APP
|- movq %rax, -88(%rbp) # pfo_ret__, %sfp
…
|-# arch/x86/include/asm/fpu/internal.h:512: return fpu == this_cpu_read_stable(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
|- movq -88(%rbp), %rcx # %sfp, pfo_ret__
|- cmpq %rcx, -64(%rbp) # pfo_ret__, %sfp
|+# arch/x86/include/asm/fpu/internal.h:512: return fpu == this_cpu_read(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
|+#APP
|+# 512 "arch/x86/include/asm/fpu/internal.h" 1
|+ movq %gs:fpu_fpregs_owner_ctx(%rip),%rax # fpu_fpregs_owner_ctx, pfo_ret__
|+# 0 "" 2
|+# arch/x86/include/asm/fpu/internal.h:512: return fpu == this_cpu_read(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
|+#NO_APP
|+ cmpq %rax, -64(%rbp) # pfo_ret__, %sfp
Use this_cpu_read() instead this_cpu_read_stable() to avoid caching of
fpu_fpregs_owner_ctx during preemption points.
Fixes: 5f409e20b7945 ("x86/fpu: Defer FPU state load until return to userspace")
---
There is no Sign-off by here. Could this please be verified by the
reporter?
Also I would like to add
Debugged-by: Ian Lance Taylor
but I lack the complete address also I'm not sure if he wants to.
Also please send a Reported-by line since I'm not sure who started this.
arch/x86/include/asm/fpu/internal.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 4c95c365058aa..44c48e34d7994 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -509,7 +509,7 @@ static inline void __fpu_invalidate_fpregs_state(struct fpu *fpu)
static inline int fpregs_state_valid(struct fpu *fpu, unsigned int cpu)
{
- return fpu == this_cpu_read_stable(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
+ return fpu == this_cpu_read(fpu_fpregs_owner_ctx) && cpu == fpu->last_cpu;
}
/*
--
2.24.0
next prev parent reply other threads:[~2019-11-27 12:42 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-26 19:49 AVX register corruption from signal delivery Barret Rhoden
2019-11-26 20:20 ` Sebastian Andrzej Siewior
2019-11-26 21:23 ` Barret Rhoden
2019-11-26 22:13 ` Borislav Petkov
2019-11-26 22:30 ` Andy Lutomirski
2019-11-26 23:00 ` Borislav Petkov
2019-11-27 12:42 ` Sebastian Andrzej Siewior [this message]
2019-11-27 14:07 ` [PATCH] x86/fpu: Don't cache access to fpu_fpregs_owner_ctx Borislav Petkov
2019-11-27 18:42 ` Barret Rhoden
2019-11-28 8:53 ` [PATCH v2] " Sebastian Andrzej Siewior
2019-11-28 9:22 ` [tip: x86/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2019-11-29 16:57 ` [PATCH v2] " David Laight
2019-11-29 17:08 ` 'Sebastian Andrzej Siewior'
2019-11-27 15:46 ` [PATCH] " Rik van Riel
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