The fault-handling code that takes mmap_sem needs to avoid a
deadlock that could occur if the kernel took a bad (OOPS-worthy)
page fault on a user address while holding mmap_sem. This can only
happen if the faulting instruction was in the kernel
(i.e. user_mode(regs)). Rather than checking the sw_error_code
(which will have the USER bit set if the fault was a USER-permission
access *or* if user_mode(regs)), just check user_mode(regs)
directly.
The old code would have malfunctioned if the kernel executed a bogus
WRUSS instruction while holding mmap_sem. Fortunately, that is
extremely unlikely in current kernels, which don't use WRUSS.
Signed-off-by: Andy Lutomirski <l...@kernel.org>
---
arch/x86/mm/fault.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 71d4b9d4d43f..91d4d2722f2e 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1344,13 +1344,10 @@ void do_user_addr_fault(struct pt_regs *regs,
* Only do the expensive exception table search when we might be at
* risk of a deadlock. This happens if we
* 1. Failed to acquire mmap_sem, and
- * 2. The access did not originate in userspace. Note: either the
- * hardware or earlier page fault code may set X86_PF_USER
- * in sw_error_code.
+ * 2. The access did not originate in userspace.
*/
if (unlikely(!down_read_trylock(&mm->mmap_sem))) {
- if (!(sw_error_code & X86_PF_USER) &&
- !search_exception_tables(regs->ip)) {
+ if (!user_mode(regs) && !search_exception_tables(regs->ip)) {
/*
* Fault from code in kernel from
* which we do not expect faults.
--
2.17.2
