[PATCH V1] fuse: Fix VM_BUG_ON_PAGE issue while accessing zero ref count page

Pradeep P V K
2020-08-20T16:56:18Z

There is a potential race between fuse_abort_conn() and
fuse_copy_page() as shown below, due to which VM_BUG_ON_PAGE
crash is observed.

context#1: context#2:
fuse_dev_do_read() fuse_abort_conn()
->fuse_copy_args() ->end_requests()
->fuse_copy_pages() ->request_end()
->fuse_copy_page() ->fuse_writepage_end)
->fuse_ref_page() ->fuse_writepage_free()
->__free_page()
->put_page_testzero()

->get_page()
->VM_BUG_ON_PAGE()

This results in below crash as when ->put_page_testzero() in context#2
decrease the page reference and get_page() in context#1 accessed it
with zero page reference count.

[ 174.391095] (1)[10406:Thread-6]page dumped because:
VM_BUG_ON_PAGE(((unsigned int) page_ref_count(page) + 127u <= 127u))
[ 174.391113] (1)[10406:Thread-6]page allocated via order 0,
migratetype Unmovable, gfp_mask
0x620042(GFP_NOFS|__GFP_HIGHMEM|__GFP_HARDWALL), pid 261, ts 174390946312 ns
[ 174.391135] (1)[10406:Thread-6] prep_new_page+0x13c/0x210
[ 174.391148] (1)[10406:Thread-6] get_page_from_freelist+0x21ac/0x2370
[ 174.391161] (1)[10406:Thread-6] __alloc_pages_nodemask+0x244/0x14a8
[ 174.391176] (1)[10406:Thread-6] fuse_writepages_fill+0x150/0x708
[ 174.391190] (1)[10406:Thread-6] write_cache_pages+0x3d8/0x550
[ 174.391202] (1)[10406:Thread-6] fuse_writepages+0x94/0x130
[ 174.391214] (1)[10406:Thread-6] do_writepages+0x74/0x140
[ 174.391228] (1)[10406:Thread-6] __writeback_single_inode+0x168/0x788
[ 174.391239] (1)[10406:Thread-6] writeback_sb_inodes+0x56c/0xab8
[ 174.391251] (1)[10406:Thread-6] __writeback_inodes_wb+0x94/0x180
[ 174.391262] (1)[10406:Thread-6] wb_writeback+0x318/0x618
[ 174.391274] (1)[10406:Thread-6] wb_workfn+0x468/0x828
[ 174.391290] (1)[10406:Thread-6] process_one_work+0x3d0/0x720
[ 174.391302] (1)[10406:Thread-6] worker_thread+0x234/0x4c0
[ 174.391314] (1)[10406:Thread-6] kthread+0x144/0x158
[ 174.391327] (1)[10406:Thread-6] ret_from_fork+0x10/0x1c
[ 174.391363] (1)[10406:Thread-6]------------[ cut here ]------------
[ 174.391371] (1)[10406:Thread-6]kernel BUG at include/linux/mm.h:980!
[ 174.391381] (1)[10406:Thread-6]Internal error: Oops - BUG: 0 [#1]
...
[ 174.486928] (1)[10406:Thread-6]pc : fuse_copy_page+0x750/0x790
[ 174.493029] (1)[10406:Thread-6]lr : fuse_copy_page+0x750/0x790
[ 174.718831] (1)[10406:Thread-6] fuse_copy_page+0x750/0x790
[ 174.718838] (1)[10406:Thread-6] fuse_copy_args+0xb4/0x1e8
[ 174.718843] (1)[10406:Thread-6] fuse_dev_do_read+0x424/0x888
[ 174.718848] (1)[10406:Thread-6] fuse_dev_splice_read+0x94/0x200
[ 174.718856] (1)[10406:Thread-6] __arm64_sys_splice+0x874/0xb20
[ 174.718864] (1)[10406:Thread-6] el0_svc_common+0xc8/0x240
[ 174.718869] (1)[10406:Thread-6] el0_svc_handler+0x6c/0x88
[ 174.718875] (1)[10406:Thread-6] el0_svc+0x8/0xc
[ 174.778853] (1)[10406:Thread-6]Kernel panic - not syncing: Fatal

Fix this by protecting fuse_copy_pages() with fc->lock.

Signed-off-by: Pradeep P V K <ppvk@codeaurora.org>
---
fs/fuse/dev.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 02b3c36..ca4f63c 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -948,22 +948,26 @@ static int fuse_copy_pages(struct fuse_copy_state *cs, unsigned nbytes,
int zeroing)
{
unsigned i;
+ int err = 0;
struct fuse_req *req = cs->req;
struct fuse_args_pages *ap = container_of(req->args, typeof(*ap), args);

-
+ if (req->ff)
+ spin_lock(&req->ff->fc->lock);
for (i = 0; i < ap->num_pages && (nbytes || zeroing); i++) {
- int err;
unsigned int offset = ap->descs[i].offset;
unsigned int count = min(nbytes, ap->descs[i].length);

err = fuse_copy_page(cs, &ap->pages[i], offset, count, zeroing);
if (err)
- return err;
+ goto err;

nbytes -= count;
}
- return 0;
+err:
+ if (req->ff)
+ spin_unlock(&req->ff->fc->lock);
+ return err;
}

/* Copy a single argument in the request to/from userspace buffer */
--
1.9.1