* [PATCH v2] exfat: fix random stack corruption after get_block
[not found] <CGME20250321063454epcas1p2194c3e69371dd4f025202d727bfb93a4@epcas1p2.samsung.com>
@ 2025-03-21 6:34 ` Sungjong Seo
2025-03-21 8:17 ` Yuezhang.Mo
2025-03-21 12:00 ` Namjae Jeon
0 siblings, 2 replies; 3+ messages in thread
From: Sungjong Seo @ 2025-03-21 6:34 UTC (permalink / raw)
To: linkinjeon, yuezhang.mo
Cc: sjdev.seo, linux-fsdevel, linux-kernel, cpgs, Sungjong Seo,
stable, Yeongjin Gil
When get_block is called with a buffer_head allocated on the stack, such
as do_mpage_readpage, stack corruption due to buffer_head UAF may occur in
the following race condition situation.
<CPU 0> <CPU 1>
mpage_read_folio
<<bh on stack>>
do_mpage_readpage
exfat_get_block
bh_read
__bh_read
get_bh(bh)
submit_bh
wait_on_buffer
...
end_buffer_read_sync
__end_buffer_read_notouch
unlock_buffer
<<keep going>>
...
...
...
...
<<bh is not valid out of mpage_read_folio>>
.
.
another_function
<<variable A on stack>>
put_bh(bh)
atomic_dec(bh->b_count)
* stack corruption here *
This patch returns -EAGAIN if a folio does not have buffers when bh_read
needs to be called. By doing this, the caller can fallback to functions
like block_read_full_folio(), create a buffer_head in the folio, and then
call get_block again.
Let's do not call bh_read() with on-stack buffer_head.
Fixes: 11a347fb6cef ("exfat: change to get file size from DataLength")
Cc: stable@vger.kernel.org
Tested-by: Yeongjin Gil <youngjin.gil@samsung.com>
Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
---
v2:
- clear_buffer_mapped if there is any errors.
- remove unnecessary BUG_ON()
---
fs/exfat/inode.c | 39 +++++++++++++++++++++++++++++++++------
1 file changed, 33 insertions(+), 6 deletions(-)
diff --git a/fs/exfat/inode.c b/fs/exfat/inode.c
index 96952d4acb50..f3fdba9f4d21 100644
--- a/fs/exfat/inode.c
+++ b/fs/exfat/inode.c
@@ -344,7 +344,8 @@ static int exfat_get_block(struct inode *inode, sector_t iblock,
* The block has been partially written,
* zero the unwritten part and map the block.
*/
- loff_t size, off, pos;
+ loff_t size, pos;
+ void *addr;
max_blocks = 1;
@@ -355,17 +356,41 @@ static int exfat_get_block(struct inode *inode, sector_t iblock,
if (!bh_result->b_folio)
goto done;
+ /*
+ * No buffer_head is allocated.
+ * (1) bmap: It's enough to fill bh_result without I/O.
+ * (2) read: The unwritten part should be filled with 0
+ * If a folio does not have any buffers,
+ * let's returns -EAGAIN to fallback to
+ * per-bh IO like block_read_full_folio().
+ */
+ if (!folio_buffers(bh_result->b_folio)) {
+ err = -EAGAIN;
+ goto done;
+ }
+
pos = EXFAT_BLK_TO_B(iblock, sb);
size = ei->valid_size - pos;
- off = pos & (PAGE_SIZE - 1);
+ addr = folio_address(bh_result->b_folio) +
+ offset_in_folio(bh_result->b_folio, pos);
+
+ /* Check if bh->b_data points to proper addr in folio */
+ if (bh_result->b_data != addr) {
+ exfat_fs_error_ratelimit(sb,
+ "b_data(%p) != folio_addr(%p)",
+ bh_result->b_data, addr);
+ err = -EINVAL;
+ goto done;
+ }
- folio_set_bh(bh_result, bh_result->b_folio, off);
+ /* Read a block */
err = bh_read(bh_result, 0);
if (err < 0)
- goto unlock_ret;
+ goto done;
- folio_zero_segment(bh_result->b_folio, off + size,
- off + sb->s_blocksize);
+ /* Zero unwritten part of a block */
+ memset(bh_result->b_data + size, 0,
+ bh_result->b_size - size);
} else {
/*
* The range has not been written, clear the mapped flag
@@ -376,6 +401,8 @@ static int exfat_get_block(struct inode *inode, sector_t iblock,
}
done:
bh_result->b_size = EXFAT_BLK_TO_B(max_blocks, sb);
+ if (err < 0)
+ clear_buffer_mapped(bh_result);
unlock_ret:
mutex_unlock(&sbi->s_lock);
return err;
--
2.25.1
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