Messages in this thread Patch in this message | | | Date | Wed, 1 May 2019 11:00:48 +0200 | From | Sebastian Andrzej Siewior <> | Subject | [RFC PATCH] x86/fpu: Use get_user_pages_unlocked() to fault-in pages |
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Using get_user_pages() seems to be problematic: KASAN reports use-after-free in LTP's signal06 testcase. The test invokes the signal handler with a provided stack and changes the RW/WO page flags of the stack while the signal is invoked. A crash due to a NULL pointer has also been observed.
get_user_pages() may be invoked (or so I assumed) without holding the mmap_sem for pre-faulting. KASAN probably slows down processing that we can observe the user-after-free while page-flags are changed. It does not happen without KASAN.
Use get_user_pages_unlocked() which holds the mm_sem around while paging-in user memory.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> ---
While this fixes the problem and the crash later on, I would like to hear from MM folks if it is intended to invoke get_user_pages() without holding the mmap_sem. Without passing lockde & pages we only do: __get_user_pages_locked() - __get_user_pages() - if (!pages) /* If it's a prefault don't insist harder */ return ret; Which was my intention. The comment above faultin_page() says "mmap_sem must be held on entry" so this makes me thing that one must hold it…
arch/x86/kernel/fpu/signal.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c index eaddb185cac95..3a94e3d2e3bdf 100644 --- a/arch/x86/kernel/fpu/signal.c +++ b/arch/x86/kernel/fpu/signal.c @@ -172,8 +172,8 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size) aligned_size = offset_in_page(buf_fx) + fpu_user_xstate_size; nr_pages = DIV_ROUND_UP(aligned_size, PAGE_SIZE); - ret = get_user_pages((unsigned long)buf_fx, nr_pages, - FOLL_WRITE, NULL, NULL); + ret = get_user_pages_unlocked((unsigned long)buf_fx, nr_pages, + NULL, FOLL_WRITE); if (ret == nr_pages) goto retry; return -EFAULT; -- 2.20.1
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