Commit-ID: 6344be608c039f3a787f1144c46fcb04c0f76561 Gitweb: https://git.kernel.org/tip/6344be608c039f3a787f1144c46fcb04c0f76561 Author: Andy Lutomirski <[email protected]> AuthorDate: Mon, 19 Nov 2018 14:45:25 -0800 Committer: Ingo Molnar <[email protected]> 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 <[email protected]> Cc: Andy Lutomirski <[email protected]> Cc: Borislav Petkov <[email protected]> Cc: Brian Gerst <[email protected]> Cc: Dave Hansen <[email protected]> Cc: Denys Vlasenko <[email protected]> Cc: H. Peter Anvin <[email protected]> Cc: Linus Torvalds <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Yu-cheng Yu <[email protected]> Link: http://lkml.kernel.org/r/4b89b542e8ceba9bd6abde2f386afed6d99244a9.1542667307.git.l...@kernel.org Signed-off-by: Ingo Molnar <[email protected]> --- 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.
