There is a cornel case that slip through the checkers in functions
reading extent buffer, ie.

if (start < eb->len) and (start + len > eb->len),
then

a) map_private_extent_buffer() returns immediately because
it's thinking the range spans across two pages,

b) and the checkers in read_extent_buffer(), WARN_ON(start > eb->len)
and WARN_ON(start + len > eb->start + eb->len), both are OK in this
corner case, but it'd actually try to access the eb->pages out of
bounds because of (start + len > eb->len).

The case is found by switching extent inline ref type from shared data
ref to non-shared data ref, which is a kind of metadata corruption.

It'd use the wrong helper to access the eb,
eg. btrfs_extent_data_ref_root(eb, ref) is used but the %ref passing
here is "struct btrfs_shared_data_ref".  And if the extent item
happens to be the first item in the eb, then offset/length will get
over eb->len which ends up an invalid memory access.

This is adding proper checks in order to avoid invalid memory access,
ie. 'general protection fault', before it's too late.

Reviewed-by: Filipe Manana <fdman...@suse.com>
Signed-off-by: Liu Bo <bo.li....@oracle.com>
---

v3: Remove the unnecessary ASSERT and num_pages.

v2: Improve the commit log to clarify that this can only happen if
metadata is corrupted.

 fs/btrfs/extent_io.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 0aff9b2..e6c6853 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -5417,8 +5417,12 @@ void read_extent_buffer(struct extent_buffer *eb, void 
*dstv,
        size_t start_offset = eb->start & ((u64)PAGE_SIZE - 1);
        unsigned long i = (start_offset + start) >> PAGE_SHIFT;
 
-       WARN_ON(start > eb->len);
-       WARN_ON(start + len > eb->start + eb->len);
+       if (start + len > eb->len) {
+               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
+                    eb->start, eb->len, start, len);
+               memset(dst, 0, len);
+               return;
+       }
 
        offset = (start_offset + start) & (PAGE_SIZE - 1);
 
@@ -5491,6 +5495,12 @@ int map_private_extent_buffer(struct extent_buffer *eb, 
unsigned long start,
        unsigned long end_i = (start_offset + start + min_len - 1) >>
                PAGE_SHIFT;
 
+       if (start + min_len > eb->len) {
+               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
+                      eb->start, eb->len, start, min_len);
+               return -EINVAL;
+       }
+
        if (i != end_i)
                return 1;
 
@@ -5502,12 +5512,6 @@ int map_private_extent_buffer(struct extent_buffer *eb, 
unsigned long start,
                *map_start = ((u64)i << PAGE_SHIFT) - start_offset;
        }
 
-       if (start + min_len > eb->len) {
-               WARN(1, KERN_ERR "btrfs bad mapping eb start %llu len %lu, 
wanted %lu %lu\n",
-                      eb->start, eb->len, start, min_len);
-               return -EINVAL;
-       }
-
        p = eb->pages[i];
        kaddr = page_address(p);
        *map = kaddr + offset;
-- 
2.9.4

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