From 75723957673bfa10c98b735259f891cc79cf0450 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 1 Oct 2007 13:17:28 -0700 Subject: [PATCH] Fix possible splice() mmap_sem deadlock Nick Piggin points out that splice isn't being good about the mmap semaphore: while two readers can nest inside each others, it does leave a possible deadlock if a writer (ie a new mmap()) comes in during that nesting. Original "just move the locking" patch by Nick, replaced by one by me based on an optimistic pagefault_disable(). And then Jens tested and updated that patch. Reported-by: Nick Piggin Tested-by: Jens Axboe Cc: Andrew Morton Signed-off-by: Linus Torvalds --- fs/splice.c | 46 ++++++++++++++++++++++++++++++++++------------ 1 file changed, 34 insertions(+), 12 deletions(-) diff --git a/fs/splice.c b/fs/splice.c index c010a72ca2d..e95a3622886 100644 --- a/fs/splice.c +++ b/fs/splice.c @@ -1223,6 +1223,33 @@ static long do_splice(struct file *in, loff_t __user *off_in, return -EINVAL; } +/* + * Do a copy-from-user while holding the mmap_semaphore for reading, in a + * manner safe from deadlocking with simultaneous mmap() (grabbing mmap_sem + * for writing) and page faulting on the user memory pointed to by src. + * This assumes that we will very rarely hit the partial != 0 path, or this + * will not be a win. + */ +static int copy_from_user_mmap_sem(void *dst, const void __user *src, size_t n) +{ + int partial; + + pagefault_disable(); + partial = __copy_from_user_inatomic(dst, src, n); + pagefault_enable(); + + /* + * Didn't copy everything, drop the mmap_sem and do a faulting copy + */ + if (unlikely(partial)) { + up_read(¤t->mm->mmap_sem); + partial = copy_from_user(dst, src, n); + down_read(¤t->mm->mmap_sem); + } + + return partial; +} + /* * Map an iov into an array of pages and offset/length tupples. With the * partial_page structure, we can map several non-contiguous ranges into @@ -1236,31 +1263,26 @@ static int get_iovec_page_array(const struct iovec __user *iov, { int buffers = 0, error = 0; - /* - * It's ok to take the mmap_sem for reading, even - * across a "get_user()". - */ down_read(¤t->mm->mmap_sem); while (nr_vecs) { unsigned long off, npages; + struct iovec entry; void __user *base; size_t len; int i; - /* - * Get user address base and length for this iovec. - */ - error = get_user(base, &iov->iov_base); - if (unlikely(error)) - break; - error = get_user(len, &iov->iov_len); - if (unlikely(error)) + error = -EFAULT; + if (copy_from_user_mmap_sem(&entry, iov, sizeof(entry))) break; + base = entry.iov_base; + len = entry.iov_len; + /* * Sanity check this iovec. 0 read succeeds. */ + error = 0; if (unlikely(!len)) break; error = -EFAULT; -- 2.41.1