Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

Peter Zijlstra
2019-02-13T18:54:17Z

On Wed, Feb 13, 2019 at 04:45:32PM +0100, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 03:41:45PM +0100, Peter Zijlstra wrote:
> > On Wed, Feb 13, 2019 at 02:39:22PM +0000, Julien Thierry wrote:
> > > Hi Peter,
> > >
> > > On 13/02/2019 14:25, Peter Zijlstra wrote:
> > > > On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
> > > >> The difference is because getting preempted in the sequence above is
> > > >> triggered off the back of an interrupt. On arm64, and I think also on x86,
> > > >> the user access state (SMAP or PAN) is saved and restored across exceptions
> > > >> but not across context switch.
> > > >
> > > > A quick reading of the SDM seems to suggest the SMAP state is part of
> > > > EFLAGS, which is context switched just fine AFAIK.
> > > >
> > > I fail to see where this is happening when looking at the switch_to()
> > > logic in x86_64.
> >
> > Yeah, me too.. we obviously preserve EFLAGS for user context, but for
> > kernel-kernel switches we do not seem to preserve it :-(
>
> So I dug around the context switch code a little, and I think we lost it
> here:
>
> 0100301bfdf5 ("sched/x86: Rewrite the switch_to() code")
>
> Before that, x86_64 switch_to() read like (much simplified):
>
> asm volatile ( /* do RSP twiddle */
> : /* output */
> : /* input */
> : "memory", "cc", .... "flags");
>
> (see __EXTRA_CLOBBER)
>
> Which I suppose means that GCC generates the PUSHF/POPF to preserve the
> EFLAGS, since we mark those explicitly clobbered.
>
> Before that:
>
> f05e798ad4c0 ("Disintegrate asm/system.h for X86")
>
> We had explicit PUSHF / POPF in SAVE_CONTEXT / RESTORE_CONTEXT resp.
>
> Now I cannot see how the current code preserves EFLAGS (if indeed it
> does), and the changelog doesn't mention this change _AT_ALL_.
>
>
> For a little bit of context; it turns out that user_access_begin() /
> user_access_end() sets EFLAGS.AC and scheduling in between there wrecks
> that because we're apparently not saving that anymore.
>
> Now, I'm tempted to add the PUSHF / POPF right back because of this, but
> first I suppose we need to figure out if that change was on purpose and
> why that went missing from the Changelog.

Just for giggles; the below patch seems to boot.

---
arch/x86/entry/entry_32.S | 2 ++
arch/x86/entry/entry_64.S | 2 ++
arch/x86/include/asm/switch_to.h | 1 +
3 files changed, 5 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index d309f30cf7af..5fc76b755510 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -650,6 +650,7 @@ ENTRY(__switch_to_asm)
pushl %ebx
pushl %edi
pushl %esi
+ pushfl

/* switch stack */
movl %esp, TASK_threadsp(%eax)
@@ -672,6 +673,7 @@ ENTRY(__switch_to_asm)
#endif

/* restore callee-saved registers */
+ popfl
popl %esi
popl %edi
popl %ebx
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 1f0efdb7b629..4fe27b67d7e2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -291,6 +291,7 @@ ENTRY(__switch_to_asm)
pushq %r13
pushq %r14
pushq %r15
+ pushfq

/* switch stack */
movq %rsp, TASK_threadsp(%rdi)
@@ -313,6 +314,7 @@ ENTRY(__switch_to_asm)
#endif

/* restore callee-saved registers */
+ popfq
popq %r15
popq %r14
popq %r13
diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
index 7cf1a270d891..157149d4129c 100644
--- a/arch/x86/include/asm/switch_to.h
+++ b/arch/x86/include/asm/switch_to.h
@@ -40,6 +40,7 @@ asmlinkage void ret_from_fork(void);
* order of the fields must match the code in __switch_to_asm().
*/
struct inactive_task_frame {
+ unsigned long flags;
#ifdef CONFIG_X86_64
unsigned long r15;
unsigned long r14;