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