Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 27 Nov 2019 15:07:54 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/fpu: Don't cache access to fpu_fpregs_owner_ctx |
| |
On Wed, Nov 27, 2019 at 01:42:43PM +0100, Sebastian Andrzej Siewior wrote: > The state/owner of FPU is saved fpu_fpregs_owner_ctx by pointing to the ^ to
> 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
Drop those "we"s :)
> 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")
Or
a352a3b7b792 ("x86/fpu: Prepare copy_fpstate_to_sigframe() for TIF_NEED_FPU_LOAD")
maybe, which adds the fpregs_unlock() ?
> --- > > There is no Sign-off by here. Could this please be verified by the > reporter?
Not the reporter, but I just tested it successfully too:
Tested-by: Borislav Petkov <bp@suse.de>
> Also I would like to add > Debugged-by: Ian Lance Taylor
Yes, pls. CCed.
> > 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; > }
And to add one more data point from IRC: this is also documented:
/* * this_cpu_read() makes gcc load the percpu variable every time it is * accessed while this_cpu_read_stable() allows the value to be cached. ^^^^^^^^^^^^^^^
* this_cpu_read_stable() is more efficient and can be used if its value * is guaranteed to be valid across cpus. The current users include * get_current() and get_thread_info() both of which are actually * per-thread variables implemented as per-cpu variables and thus * stable for the duration of the respective task. */ #define this_cpu_read_stable(var) percpu_stable_op("mov", var)
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
| ![\](/images/icornerr.gif) |