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 --- fs/exfat/inode.c | 37 ++++++++++++++++++++++++++++++++----- 1 file changed, 32 insertions(+), 5 deletions(-)

diff --git a/fs/exfat/inode.c b/fs/exfat/inode.c index 96952d4acb50..b8ea910586e4 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 +356,43 @@ 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); + + BUG_ON(size > sb->s_blocksize); + + /* 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;

- 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