2023-08-09 08:23:59

by GONG, Ruiqi

Subject: [PATCH v3] netfilter: ebtables: fix fortify warnings in size_entry_mwt()

From: "GONG, Ruiqi" <[email protected]>

When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
warning appears:

In function ‘fortify_memcpy_chk’,
inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
declared with attribute warning: detected read beyond size of field (2nd parameter);
maybe use struct_group()? [-Werror=attribute-warning]
592 | __read_overflow2_field(q_size_field, size);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The compiler is complaining:

memcpy(&offsets[1], &entry->watchers_offset,
sizeof(offsets) - sizeof(offsets[0]));

where memcpy reads beyong &entry->watchers_offset to copy
{watchers,target,next}_offset altogether into offsets[]. Silence the
warning by wrapping these three up via struct_group().

Signed-off-by: GONG, Ruiqi <[email protected]>
Reviewed-by: Gustavo A. R. Silva <[email protected]>
---

v3: use sizeof(entry->offsets) as memcpy()'s size, as Gustavo suggested.

v2: fix HDRTEST error by replacing struct_group() with __struct_group(),
since it's a uapi header.

include/uapi/linux/netfilter_bridge/ebtables.h | 14 ++++++++------
net/bridge/netfilter/ebtables.c | 3 +--
2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/include/uapi/linux/netfilter_bridge/ebtables.h b/include/uapi/linux/netfilter_bridge/ebtables.h
index a494cf43a755..b0caad82b693 100644
--- a/include/uapi/linux/netfilter_bridge/ebtables.h
+++ b/include/uapi/linux/netfilter_bridge/ebtables.h
@@ -182,12 +182,14 @@ struct ebt_entry {
unsigned char sourcemsk[ETH_ALEN];
unsigned char destmac[ETH_ALEN];
unsigned char destmsk[ETH_ALEN];
- /* sizeof ebt_entry + matches */
- unsigned int watchers_offset;
- /* sizeof ebt_entry + matches + watchers */
- unsigned int target_offset;
- /* sizeof ebt_entry + matches + watchers + target */
- unsigned int next_offset;
+ __struct_group(/* no tag */, offsets, /* no attrs */,
+ /* sizeof ebt_entry + matches */
+ unsigned int watchers_offset;
+ /* sizeof ebt_entry + matches + watchers */
+ unsigned int target_offset;
+ /* sizeof ebt_entry + matches + watchers + target */
+ unsigned int next_offset;
+ );
unsigned char elems[0] __attribute__ ((aligned (__alignof__(struct ebt_replace))));
};

diff --git a/net/bridge/netfilter/ebtables.c b/net/bridge/netfilter/ebtables.c
index 757ec46fc45a..aa23479b20b2 100644
--- a/net/bridge/netfilter/ebtables.c
+++ b/net/bridge/netfilter/ebtables.c
@@ -2115,8 +2115,7 @@ static int size_entry_mwt(const struct ebt_entry *entry, const unsigned char *ba
return ret;

offsets[0] = sizeof(struct ebt_entry); /* matches come first */
- memcpy(&offsets[1], &entry->watchers_offset,
- sizeof(offsets) - sizeof(offsets[0]));
+ memcpy(&offsets[1], &entry->offsets, sizeof(entry->offsets));

if (state->buf_kern_start) {
buf_start = state->buf_kern_start + state->buf_kern_offset;
--
2.41.0

2023-08-09 12:54:08

by Simon Horman

Subject: Re: [PATCH v3] netfilter: ebtables: fix fortify warnings in size_entry_mwt()

On Wed, Aug 09, 2023 at 03:45:03PM +0800, GONG, Ruiqi wrote:
> From: "GONG, Ruiqi" <[email protected]>
>
> When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
> warning appears:
>
> In function ‘fortify_memcpy_chk’,
> inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
> ./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
> declared with attribute warning: detected read beyond size of field (2nd parameter);
> maybe use struct_group()? [-Werror=attribute-warning]
> 592 | __read_overflow2_field(q_size_field, size);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The compiler is complaining:
>
> memcpy(&offsets[1], &entry->watchers_offset,
> sizeof(offsets) - sizeof(offsets[0]));
>
> where memcpy reads beyong &entry->watchers_offset to copy

nit: beyong -> beyond

...

2023-08-10 21:54:25

by Kees Cook

Subject: Re: [PATCH v3] netfilter: ebtables: fix fortify warnings in size_entry_mwt()

On Wed, Aug 09, 2023 at 03:45:03PM +0800, GONG, Ruiqi wrote:
> From: "GONG, Ruiqi" <[email protected]>
>
> When compiling with gcc 13 and CONFIG_FORTIFY_SOURCE=y, the following
> warning appears:
>
> In function ‘fortify_memcpy_chk’,
> inlined from ‘size_entry_mwt’ at net/bridge/netfilter/ebtables.c:2118:2:
> ./include/linux/fortify-string.h:592:25: error: call to ‘__read_overflow2_field’
> declared with attribute warning: detected read beyond size of field (2nd parameter);
> maybe use struct_group()? [-Werror=attribute-warning]
> 592 | __read_overflow2_field(q_size_field, size);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> The compiler is complaining:
>
> memcpy(&offsets[1], &entry->watchers_offset,
> sizeof(offsets) - sizeof(offsets[0]));
>
> where memcpy reads beyong &entry->watchers_offset to copy
> {watchers,target,next}_offset altogether into offsets[]. Silence the
> warning by wrapping these three up via struct_group().
>
> Signed-off-by: GONG, Ruiqi <[email protected]>

If a v4 is sent, please fix the "beyong" typo that was pointed out.
Otherwise, it looks okay to me:

Reviewed-by: Kees Cook <[email protected]>

--
Kees Cook