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

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:

quoted

On Wed, Feb 13, 2019 at 02:39:22PM +0000, Julien Thierry wrote:

quoted

Hi Peter,

On 13/02/2019 14:25, Peter Zijlstra wrote:

quoted

On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:

quoted

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;

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