[PATCH v2 04/10] x86/entry/32: Restore FLAGS on SYSEXIT

[Andy Lutomirski]

We weren't restoring FLAGS at all on SYSEXIT.  Apparently no one cared.

With this patch applied, native kernels should always honor
task_pt_regs()->flags, which opens the door for some sys_iopl
cleanups.  I'll do those as a separate series, though, since getting
it right will involve tweaking some paravirt ops.

(The short version is that, before this patch, sys_iopl, invoked via
 SYSENTER, wasn't guaranteed to ever transfer the updated
 regs->flags, so sys_iopl had to change the hardware flags register
 as well.)

Reported-by: Brian Gerst 
Signed-off-by: Andy Lutomirski 
---
 arch/x86/entry/entry_32.S | 9 +
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 289a17bf0c71..a8c3424c3392 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -343,6 +343,15 @@ sysenter_past_esp:
popl%eax/* pt_regs->ax */
 
/*
+* Restore all flags except IF (we restore IF separately because
+* STI gives a one-instruction window in which we won't be interrupted,
+* whereas POPF does not.
+*/
+   addl$PT_EFLAGS-PT_DS, %esp  /* point esp at pt_regs->flags */
+   btr $X86_EFLAGS_IF_BIT, (%esp)
+   popfl
+
+   /*
 * Return back to the vDSO, which will pop ecx and edx.
 * Don't bother with DS and ES (they already contain __USER_DS).
 */
-- 
2.5.0

[PATCH v2 04/10] x86/entry/32: Restore FLAGS on SYSEXIT

[Andy Lutomirski]

We weren't restoring FLAGS at all on SYSEXIT.  Apparently no one cared.

With this patch applied, native kernels should always honor
task_pt_regs()->flags, which opens the door for some sys_iopl
cleanups.  I'll do those as a separate series, though, since getting
it right will involve tweaking some paravirt ops.

(The short version is that, before this patch, sys_iopl, invoked via
 SYSENTER, wasn't guaranteed to ever transfer the updated
 regs->flags, so sys_iopl had to change the hardware flags register
 as well.)

Reported-by: Brian Gerst 
Signed-off-by: Andy Lutomirski 
---
 arch/x86/entry/entry_32.S | 9 +
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 289a17bf0c71..a8c3424c3392 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -343,6 +343,15 @@ sysenter_past_esp:
popl%eax/* pt_regs->ax */
 
/*
+* Restore all flags except IF (we restore IF separately because
+* STI gives a one-instruction window in which we won't be interrupted,
+* whereas POPF does not.
+*/
+   addl$PT_EFLAGS-PT_DS, %esp  /* point esp at pt_regs->flags */
+   btr $X86_EFLAGS_IF_BIT, (%esp)
+   popfl
+
+   /*
 * Return back to the vDSO, which will pop ecx and edx.
 * Don't bother with DS and ES (they already contain __USER_DS).
 */
-- 
2.5.0