From: Vineet Gupta vgupta@synopsys.com
[ Upstream commit 4d447455e73b47c43dd35fcc38ed823d3182a474 ]
do_page_fault() forgot to relinquish mmap_sem if a signal came while handling handle_mm_fault() - due to say a ctl+c or oom etc. This would later cause a deadlock by acquiring it twice.
This came to light when running libc testsuite tst-tls3-malloc test but is likely also the cause for prior seen LTP failures. Using lockdep clearly showed what the issue was.
| # while true; do ./tst-tls3-malloc ; done | Didn't expect signal from child: got `Segmentation fault' | ^C | ============================================ | WARNING: possible recursive locking detected | 4.17.0+ #25 Not tainted | -------------------------------------------- | tst-tls3-malloc/510 is trying to acquire lock: | 606c7728 (&mm->mmap_sem){++++}, at: __might_fault+0x28/0x5c | |but task is already holding lock: |606c7728 (&mm->mmap_sem){++++}, at: do_page_fault+0x9c/0x2a0 | | other info that might help us debug this: | Possible unsafe locking scenario: | | CPU0 | ---- | lock(&mm->mmap_sem); | lock(&mm->mmap_sem); | | *** DEADLOCK *** |
------------------------------------------------------------ What the change does is not obvious (note to myself)
prior code was
| do_page_fault | | down_read() <-- lock taken | handle_mm_fault <-- signal pending as this runs | if fatal_signal_pending | if VM_FAULT_ERROR | up_read | if user_mode | return <-- lock still held, this was the BUG
New code
| do_page_fault | | down_read() <-- lock taken | handle_mm_fault <-- signal pending as this runs | if fatal_signal_pending | if VM_FAULT_RETRY | return <-- not same case as above, but still OK since | core mm already relinq lock for FAULT_RETRY | ... | | < Now falls through for bug case above > | | up_read() <-- lock relinquished
Cc: stable@vger.kernel.org Signed-off-by: Vineet Gupta vgupta@synopsys.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/arc/mm/fault.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)
diff --git a/arch/arc/mm/fault.c b/arch/arc/mm/fault.c index db6913094be3c..f28db0b112a30 100644 --- a/arch/arc/mm/fault.c +++ b/arch/arc/mm/fault.c @@ -143,12 +143,17 @@ void do_page_fault(unsigned long address, struct pt_regs *regs) */ fault = handle_mm_fault(vma, address, flags);
- /* If Pagefault was interrupted by SIGKILL, exit page fault "early" */ if (unlikely(fatal_signal_pending(current))) { - if ((fault & VM_FAULT_ERROR) && !(fault & VM_FAULT_RETRY)) - up_read(&mm->mmap_sem); - if (user_mode(regs)) + + /* + * if fault retry, mmap_sem already relinquished by core mm + * so OK to return to user mode (with signal handled first) + */ + if (fault & VM_FAULT_RETRY) { + if (!user_mode(regs)) + goto no_context; return; + } }
perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);