Re: [PATCH v3] Btrfs: fix out of bounds array access while reading extent buffer

[Duncan]

Hugo Mills posted on Wed, 09 Aug 2017 18:15:29 + as excerpted:

> On Wed, Aug 09, 2017 at 11:10:16AM -0600, Liu Bo wrote:
>> There is a cornel case that slip through the checkers in functions
>  ^^ corner
> 
>Sorry, that's been bugging me every time it goes past. A cornel is
> a kind of tree, apparently.

LOL!  Me too but likely more so because I actually live in Cornell 
Condominiums so seeing "cornel" in this context is a rather confusing 
context switch, to say the least!  I let it slip the first time as I 
never expected it to make it to a v3, but...

-- 
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and if you use the program, he is your master."  Richard Stallman

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Re: [PATCH v3] Btrfs: fix out of bounds array access while reading extent buffer

[Hugo Mills]

On Wed, Aug 09, 2017 at 11:10:16AM -0600, Liu Bo wrote:
> There is a cornel case that slip through the checkers in functions
 ^^ corner

   Sorry, that's been bugging me every time it goes past. A cornel is
a kind of tree, apparently.

   Hugo.

> 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 
> Signed-off-by: Liu Bo 
> ---
> 
> 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;

-- 
Hugo Mills | Jazz is the sort of music where no-one plays
hugo@... carfax.org.uk | anything the same way once.
http://carfax.org.uk/  |
PGP: E2AB1DE4  |


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[PATCH v3] Btrfs: fix out of bounds array access while reading extent buffer

[Liu Bo]

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 
Signed-off-by: Liu Bo 
---

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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