pipe: Fix missing mask update after pipe_wait()

Fix pipe_write() to not cache the ring index mask and max_usage as their
values are invalidated by calling pipe_wait() because the latter
function drops the pipe lock, thereby allowing F_SETPIPE_SZ change them.
Without this, pipe_write() may subsequently miscalculate the array
indices and pipe fullness, leading to an oops like the following:

  BUG: KASAN: slab-out-of-bounds in pipe_write+0xc25/0xe10 fs/pipe.c:481
  Write of size 8 at addr ffff8880771167a8 by task syz-executor.3/7987
  ...
  CPU: 1 PID: 7987 Comm: syz-executor.3 Not tainted 5.4.0-rc2-syzkaller #0
  ...
  Call Trace:
    pipe_write+0xc25/0xe10 fs/pipe.c:481
    call_write_iter include/linux/fs.h:1895 [inline]
    new_sync_write+0x3fd/0x7e0 fs/read_write.c:483
    __vfs_write+0x94/0x110 fs/read_write.c:496
    vfs_write+0x18a/0x520 fs/read_write.c:558
    ksys_write+0x105/0x220 fs/read_write.c:611
    __do_sys_write fs/read_write.c:623 [inline]
    __se_sys_write fs/read_write.c:620 [inline]
    __x64_sys_write+0x6e/0xb0 fs/read_write.c:620
    do_syscall_64+0xca/0x5d0 arch/x86/entry/common.c:290
    entry_SYSCALL_64_after_hwframe+0x49/0xbe

This is not a problem for pipe_read() as the mask is recalculated on
each pass of the loop, after pipe_wait() has been called.

Fixes: 8cefc107ca54 ("pipe: Use head and tail pointers for the ring, not cursor and length")
Reported-by: syzbot+838eb0878ffd51f27c41@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
Cc: Eric Biggers <ebiggers@kernel.org>
[ Changed it to use a temporary variable 'mask' to avoid long lines -Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/pipe.c b/fs/pipe.c
index 05330fa..70313af 100644 (file)
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -389,7 +389,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
 {
        struct file *filp = iocb->ki_filp;
        struct pipe_inode_info *pipe = filp->private_data;
-       unsigned int head, max_usage, mask;
+       unsigned int head;
        ssize_t ret = 0;
        int do_wakeup = 0;
        size_t total_len = iov_iter_count(from);
@@ -408,12 +408,11 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
        }
 
        head = pipe->head;
-       max_usage = pipe->max_usage;
-       mask = pipe->ring_size - 1;
 
        /* We try to merge small writes */
        chars = total_len & (PAGE_SIZE-1); /* size of the last buffer */
        if (!pipe_empty(head, pipe->tail) && chars != 0) {
+               unsigned int mask = pipe->ring_size - 1;
                struct pipe_buffer *buf = &pipe->bufs[(head - 1) & mask];
                int offset = buf->offset + buf->len;
 
@@ -443,7 +442,8 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
                }
 
                head = pipe->head;
-               if (!pipe_full(head, pipe->tail, max_usage)) {
+               if (!pipe_full(head, pipe->tail, pipe->max_usage)) {
+                       unsigned int mask = pipe->ring_size - 1;
                        struct pipe_buffer *buf = &pipe->bufs[head & mask];
                        struct page *page = pipe->tmp_page;
                        int copied;
@@ -465,7 +465,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
                        spin_lock_irq(&pipe->wait.lock);
 
                        head = pipe->head;
-                       if (pipe_full(head, pipe->tail, max_usage)) {
+                       if (pipe_full(head, pipe->tail, pipe->max_usage)) {
                                spin_unlock_irq(&pipe->wait.lock);
                                continue;
                        }
@@ -510,7 +510,7 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
                                break;
                }
 
-               if (!pipe_full(head, pipe->tail, max_usage))
+               if (!pipe_full(head, pipe->tail, pipe->max_usage))
                        continue;
 
                /* Wait for buffer space to become available. */