exfat: fix random stack corruption after get_block

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>
Reviewed-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

diff --git a/fs/exfat/inode.c b/fs/exfat/inode.c
index 96952d4acb500f8e103571beeb43a9840858bd08..f3fdba9f4d21a343c4383282835e17d446f8b2d4 100644 (file)
--- 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;