I think you want these too:
af726f21ed8a x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C
b645af2d5905 x86_64, traps: Rework bad_iret
I'm attaching backports to 3.2.
Ben.
--
Ben Hutchings
Experience is directly proportional to the value of equipment destroyed.
- Carolyn Scheppner
From: Andy Lutomirski <luto@amacapital.net>
Date: Sat, 22 Nov 2014 18:00:33 -0800
Subject: x86_64, traps: Rework bad_iret
commit b645af2d5905c4e32399005b867987919cbfc3ae upstream.
It's possible for iretq to userspace to fail. This can happen because
of a bad CS, SS, or RIP.
Historically, we've handled it by fixing up an exception from iretq to
land at bad_iret, which pretends that the failed iret frame was really
the hardware part of #GP(0) from userspace. To make this work, there's
an extra fixup to fudge the gs base into a usable state.
This is suboptimal because it loses the original exception. It's also
buggy because there's no guarantee that we were on the kernel stack to
begin with. For example, if the failing iret happened on return from an
NMI, then we'll end up executing general_protection on the NMI stack.
This is bad for several reasons, the most immediate of which is that
general_protection, as a non-paranoid idtentry, will try to deliver
signals and/or schedule from the wrong stack.
This patch throws out bad_iret entirely. As a replacement, it augments
the existing swapgs fudge into a full-blown iret fixup, mostly written
in C. It's should be clearer and more correct.
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: we didn't use the _ASM_EXTABLE macro]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -875,12 +875,14 @@ ENTRY(native_iret)
.global native_irq_return_iret
native_irq_return_iret:
+ /*
+ * This may fault. Non-paranoid faults on return to userspace are
+ * handled by fixup_bad_iret. These include #SS, #GP, and #NP.
+ * Double-faults due to espfix64 are handled in do_double_fault.
+ * Other faults here are fatal.
+ */
iretq
- .section __ex_table,"a"
- .quad native_irq_return_iret, bad_iret
- .previous
-
#ifdef CONFIG_X86_ESPFIX64
native_irq_return_ldt:
pushq_cfi %rax
@@ -907,25 +909,6 @@ native_irq_return_ldt:
jmp native_irq_return_iret
#endif
- .section .fixup,"ax"
-bad_iret:
- /*
- * The iret traps when the %cs or %ss being restored is bogus.
- * We've lost the original trap vector and error code.
- * #GPF is the most likely one to get for an invalid selector.
- * So pretend we completed the iret and took the #GPF in user mode.
- *
- * We are now running with the kernel GS after exception recovery.
- * But error_entry expects us to have user GS to match the user %cs,
- * so swap back.
- */
- pushq $0
-
- SWAPGS
- jmp general_protection
-
- .previous
-
/* edi: workmask, edx: work */
retint_careful:
CFI_RESTORE_STATE
@@ -1463,16 +1446,15 @@ error_sti:
/*
* There are two places in the kernel that can potentially fault with
- * usergs. Handle them here. The exception handlers after iret run with
- * kernel gs again, so don't set the user space flag. B stepping K8s
- * sometimes report an truncated RIP for IRET exceptions returning to
- * compat mode. Check for these here too.
+ * usergs. Handle them here. B stepping K8s sometimes report a
+ * truncated RIP for IRET exceptions returning to compat mode. Check
+ * for these here too.
*/
error_kernelspace:
incl %ebx
leaq native_irq_return_iret(%rip),%rcx
cmpq %rcx,RIP+8(%rsp)
- je error_swapgs
+ je error_bad_iret
movl %ecx,%eax /* zero extend */
cmpq %rax,RIP+8(%rsp)
je bstep_iret
@@ -1483,7 +1465,15 @@ error_kernelspace:
bstep_iret:
/* Fix truncated RIP */
movq %rcx,RIP+8(%rsp)
- jmp error_swapgs
+ /* fall through */
+
+error_bad_iret:
+ SWAPGS
+ mov %rsp,%rdi
+ call fixup_bad_iret
+ mov %rax,%rsp
+ decl %ebx /* Return to usergs */
+ jmp error_sti
CFI_ENDPROC
END(error_entry)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -363,6 +363,35 @@ asmlinkage __kprobes struct pt_regs *syn
*regs = *eregs;
return regs;
}
+
+struct bad_iret_stack {
+ void *error_entry_ret;
+ struct pt_regs regs;
+};
+
+asmlinkage
+struct bad_iret_stack *fixup_bad_iret(struct bad_iret_stack *s)
+{
+ /*
+ * This is called from entry_64.S early in handling a fault
+ * caused by a bad iret to user mode. To handle the fault
+ * correctly, we want move our stack frame to task_pt_regs
+ * and we want to pretend that the exception came from the
+ * iret target.
+ */
+ struct bad_iret_stack *new_stack =
+ container_of(task_pt_regs(current),
+ struct bad_iret_stack, regs);
+
+ /* Copy the IRET target to the new stack. */
+ memmove(&new_stack->regs.ip, (void *)s->regs.sp, 5*8);
+
+ /* Copy the remainder of the stack from the current stack. */
+ memmove(new_stack, s, offsetof(struct bad_iret_stack, regs.ip));
+
+ BUG_ON(!user_mode_vm(&new_stack->regs));
+ return new_stack;
+}
#endif
/*
From: Andy Lutomirski <luto@amacapital.net>
Date: Sat, 22 Nov 2014 18:00:31 -0800
Subject: x86_64, traps: Fix the espfix64 #DF fixup and rewrite it in C
commit af726f21ed8af2cdaa4e93098dc211521218ae65 upstream.
There's nothing special enough about the espfix64 double fault fixup to
justify writing it in assembly. Move it to C.
This also fixes a bug: if the double fault came from an IST stack, the
old asm code would return to a partially uninitialized stack frame.
Fixes: 3891a04aafd668686239349ea58f3314ea2af86b
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2:
- Keep using the paranoiderrorentry macro to generate the asm code
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kernel/entry_64.S | 34 ++--------------------------------
arch/x86/kernel/traps.c | 24 ++++++++++++++++++++++++
2 files changed, 26 insertions(+), 32 deletions(-)
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -873,6 +873,7 @@ ENTRY(native_iret)
jnz native_irq_return_ldt
#endif
+.global native_irq_return_iret
native_irq_return_iret:
iretq
@@ -972,37 +973,6 @@ ENTRY(retint_kernel)
CFI_ENDPROC
END(common_interrupt)
- /*
- * If IRET takes a fault on the espfix stack, then we
- * end up promoting it to a doublefault. In that case,
- * modify the stack to make it look like we just entered
- * the #GP handler from user space, similar to bad_iret.
- */
-#ifdef CONFIG_X86_ESPFIX64
- ALIGN
-__do_double_fault:
- XCPT_FRAME 1 RDI+8
- movq RSP(%rdi),%rax /* Trap on the espfix stack? */
- sarq $PGDIR_SHIFT,%rax
- cmpl $ESPFIX_PGD_ENTRY,%eax
- jne do_double_fault /* No, just deliver the fault */
- cmpl $__KERNEL_CS,CS(%rdi)
- jne do_double_fault
- movq RIP(%rdi),%rax
- cmpq $native_irq_return_iret,%rax
- jne do_double_fault /* This shouldn't happen... */
- movq PER_CPU_VAR(kernel_stack),%rax
- subq $(6*8-KERNEL_STACK_OFFSET),%rax /* Reset to original stack */
- movq %rax,RSP(%rdi)
- movq $0,(%rax) /* Missing (lost) #GP error code */
- movq $general_protection,RIP(%rdi)
- retq
- CFI_ENDPROC
-END(__do_double_fault)
-#else
-# define __do_double_fault do_double_fault
-#endif
-
/*
* End of kprobes section
*/
@@ -1169,7 +1139,7 @@ zeroentry overflow do_overflow
zeroentry bounds do_bounds
zeroentry invalid_op do_invalid_op
zeroentry device_not_available do_device_not_available
-paranoiderrorentry double_fault __do_double_fault
+paranoiderrorentry double_fault do_double_fault
zeroentry coprocessor_segment_overrun do_coprocessor_segment_overrun
errorentry invalid_TSS do_invalid_TSS
errorentry segment_not_present do_segment_not_present
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -224,6 +224,30 @@ dotraplinkage void do_double_fault(struc
static const char str[] = "double fault";
struct task_struct *tsk = current;
+#ifdef CONFIG_X86_ESPFIX64
+ extern unsigned char native_irq_return_iret[];
+
+ /*
+ * If IRET takes a non-IST fault on the espfix64 stack, then we
+ * end up promoting it to a doublefault. In that case, modify
+ * the stack to make it look like we just entered the #GP
+ * handler from user space, similar to bad_iret.
+ */
+ if (((long)regs->sp >> PGDIR_SHIFT) == ESPFIX_PGD_ENTRY &&
+ regs->cs == __KERNEL_CS &&
+ regs->ip == (unsigned long)native_irq_return_iret)
+ {
+ struct pt_regs *normal_regs = task_pt_regs(current);
+
+ /* Fake a #GP(0) from userspace. */
+ memmove(&normal_regs->ip, (void *)regs->sp, 5*8);
+ normal_regs->orig_ax = 0; /* Missing (lost) #GP error code */
+ regs->ip = (unsigned long)general_protection;
+ regs->sp = (unsigned long)&normal_regs->orig_ax;
+ return;
+ }
+#endif
+
/* Return not checked because double check cannot be ignored */
notify_die(DIE_TRAP, str, regs, error_code, X86_TRAP_DF, SIGSEGV);
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