Commit-ID: 6344be608c039f3a787f1144c46fcb04c0f76561
Gitweb: https://git.kernel.org/tip/6344be608c039f3a787f1144c46fcb04c0f76561
Author: Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate: Mon, 19 Nov 2018 14:45:25 -0800
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 20 Nov 2018 08:44:27 +0100
x86/fault: Check user_mode(regs) when avoiding an mmap_sem deadlock
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 <luto@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
Link: http://lkml.kernel.org/r/4b89b542e8ceba9bd6abde2f386afed6d99244a9.1542667307.git.luto@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
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 29525cf21100..8624cb7d8d65 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.
|