* [PATCH] iommufd: Add top-level bounds check on kernel buffer size
@ 2023-01-27 22:38 Kees Cook
2023-01-28 0:47 ` Jason Gunthorpe
0 siblings, 1 reply; 5+ messages in thread
From: Kees Cook @ 2023-01-27 22:38 UTC (permalink / raw)
To: Jason Gunthorpe
Cc: Kees Cook, Kevin Tian, Joerg Roedel, Will Deacon, Robin Murphy,
iommu, linux-kernel, linux-hardening
While the op->size assignments are already bounds-checked at static
initializer time, these limits aren't aggregated and tracked when doing
later variable range checking under -Warray-bounds. Help the compiler
see that we know what we're talking about, and we'll never ask to
write more that sizeof(ucmd.cmd) bytes during the memset() inside
copy_struct_from_user(). Seen under GCC 13:
In function 'copy_struct_from_user',
inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
59 | #define __underlying_memset __builtin_memset
| ^
../include/linux/fortify-string.h:453:9: note: in expansion of macro '__underlying_memset'
453 | __underlying_memset(p, c, __fortify_size); \
| ^~~~~~~~~~~~~~~~~~~
../include/linux/fortify-string.h:461:25: note: in expansion of macro '__fortify_memset_chk'
461 | #define memset(p, c, s) __fortify_memset_chk(p, c, s, \
| ^~~~~~~~~~~~~~~~~~~~
../include/linux/uaccess.h:334:17: note: in expansion of macro 'memset'
334 | memset(dst + size, 0, rest);
| ^~~~~~
../drivers/iommu/iommufd/main.c: In function 'iommufd_fops_ioctl':
../drivers/iommu/iommufd/main.c:311:27: note: 'buf' declared here
311 | union ucmd_buffer buf;
| ^~~
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Kevin Tian <kevin.tian@intel.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Will Deacon <will@kernel.org>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: iommu@lists.linux.dev
Signed-off-by: Kees Cook <keescook@chromium.org>
---
drivers/iommu/iommufd/main.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/drivers/iommu/iommufd/main.c b/drivers/iommu/iommufd/main.c
index 3fbe636c3d8a..34a1785da33a 100644
--- a/drivers/iommu/iommufd/main.c
+++ b/drivers/iommu/iommufd/main.c
@@ -330,8 +330,9 @@ static long iommufd_fops_ioctl(struct file *filp, unsigned int cmd,
return -EINVAL;
ucmd.cmd = &buf;
- ret = copy_struct_from_user(ucmd.cmd, op->size, ucmd.ubuffer,
- ucmd.user_size);
+ ret = copy_struct_from_user(ucmd.cmd,
+ min_t(size_t, op->size, sizeof(ucmd.cmd)),
+ ucmd.ubuffer, ucmd.user_size);
if (ret)
return ret;
ret = op->execute(&ucmd);
--
2.34.1
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size
2023-01-27 22:38 [PATCH] iommufd: Add top-level bounds check on kernel buffer size Kees Cook
@ 2023-01-28 0:47 ` Jason Gunthorpe
2023-01-28 0:57 ` Kees Cook
0 siblings, 1 reply; 5+ messages in thread
From: Jason Gunthorpe @ 2023-01-28 0:47 UTC (permalink / raw)
To: Kees Cook
Cc: Kevin Tian, Joerg Roedel, Will Deacon, Robin Murphy, iommu,
linux-kernel, linux-hardening
On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote:
> While the op->size assignments are already bounds-checked at static
> initializer time, these limits aren't aggregated and tracked when doing
> later variable range checking under -Warray-bounds. Help the compiler
> see that we know what we're talking about, and we'll never ask to
> write more that sizeof(ucmd.cmd) bytes during the memset() inside
> copy_struct_from_user(). Seen under GCC 13:
>
> In function 'copy_struct_from_user',
> inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
> ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
> 59 | #define __underlying_memset __builtin_memset
This seems strange to me
I thought the way gcc handled this was if it knew the value must be in
a certain range then it would check it
If it couldn't figure out any ranges it would not make a warning.
So why did it decide "rest" was in that really weird range?
Is this just a compiler bug?
Jason
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size
2023-01-28 0:47 ` Jason Gunthorpe
@ 2023-01-28 0:57 ` Kees Cook
2023-01-28 1:13 ` Jason Gunthorpe
0 siblings, 1 reply; 5+ messages in thread
From: Kees Cook @ 2023-01-28 0:57 UTC (permalink / raw)
To: Jason Gunthorpe
Cc: Kevin Tian, Joerg Roedel, Will Deacon, Robin Murphy, iommu,
linux-kernel, linux-hardening
On Fri, Jan 27, 2023 at 08:47:34PM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote:
> > While the op->size assignments are already bounds-checked at static
> > initializer time, these limits aren't aggregated and tracked when doing
> > later variable range checking under -Warray-bounds. Help the compiler
> > see that we know what we're talking about, and we'll never ask to
> > write more that sizeof(ucmd.cmd) bytes during the memset() inside
> > copy_struct_from_user(). Seen under GCC 13:
> >
> > In function 'copy_struct_from_user',
> > inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
> > ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
> > 59 | #define __underlying_memset __builtin_memset
>
> This seems strange to me
>
> I thought the way gcc handled this was if it knew the value must be in
> a certain range then it would check it
>
> If it couldn't figure out any ranges it would not make a warning.
>
> So why did it decide "rest" was in that really weird range?
It's because it got bounds-checked at the lower end (for the minimum
size test).
>
> Is this just a compiler bug?
I don't think so. This just keeps the bounds within the buffer size now.
--
Kees Cook
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size
2023-01-28 0:57 ` Kees Cook
@ 2023-01-28 1:13 ` Jason Gunthorpe
2023-02-01 1:01 ` Kees Cook
0 siblings, 1 reply; 5+ messages in thread
From: Jason Gunthorpe @ 2023-01-28 1:13 UTC (permalink / raw)
To: Kees Cook
Cc: Kevin Tian, Joerg Roedel, Will Deacon, Robin Murphy, iommu,
linux-kernel, linux-hardening
On Fri, Jan 27, 2023 at 04:57:26PM -0800, Kees Cook wrote:
> On Fri, Jan 27, 2023 at 08:47:34PM -0400, Jason Gunthorpe wrote:
> > On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote:
> > > While the op->size assignments are already bounds-checked at static
> > > initializer time, these limits aren't aggregated and tracked when doing
> > > later variable range checking under -Warray-bounds. Help the compiler
> > > see that we know what we're talking about, and we'll never ask to
> > > write more that sizeof(ucmd.cmd) bytes during the memset() inside
> > > copy_struct_from_user(). Seen under GCC 13:
> > >
> > > In function 'copy_struct_from_user',
> > > inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
> > > ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
> > > 59 | #define __underlying_memset __builtin_memset
> >
> > This seems strange to me
> >
> > I thought the way gcc handled this was if it knew the value must be in
> > a certain range then it would check it
> >
> > If it couldn't figure out any ranges it would not make a warning.
> >
> > So why did it decide "rest" was in that really weird range?
>
> It's because it got bounds-checked at the lower end (for the minimum
> size test).
Where? There is no sizeof(ucmd.ubuffer) in this code.
There are no statically computable constants at all.
The minimum size test loads from a struct:
if (ucmd.user_size < op->min_size)
return -EINVAL;
So, either gcc can't see through that and thus has no idea what the
bound check is
Or, gcc has figured out that struct iommufd_ioctl_op::min_size has a
finite set of values
If the latter, why doesn't it also know that iommufd_ioctl_op::size
has finite set too?
Combined with the weird report that the upper end of that range is -2
(not UINT_MAX), something very strange is going on inside gcc.
Jason
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH] iommufd: Add top-level bounds check on kernel buffer size
2023-01-28 1:13 ` Jason Gunthorpe
@ 2023-02-01 1:01 ` Kees Cook
0 siblings, 0 replies; 5+ messages in thread
From: Kees Cook @ 2023-02-01 1:01 UTC (permalink / raw)
To: Jason Gunthorpe
Cc: Kevin Tian, Joerg Roedel, Will Deacon, Robin Murphy, iommu,
linux-kernel, linux-hardening
On Fri, Jan 27, 2023 at 09:13:31PM -0400, Jason Gunthorpe wrote:
> On Fri, Jan 27, 2023 at 04:57:26PM -0800, Kees Cook wrote:
> > On Fri, Jan 27, 2023 at 08:47:34PM -0400, Jason Gunthorpe wrote:
> > > On Fri, Jan 27, 2023 at 02:38:17PM -0800, Kees Cook wrote:
> > > > While the op->size assignments are already bounds-checked at static
> > > > initializer time, these limits aren't aggregated and tracked when doing
> > > > later variable range checking under -Warray-bounds. Help the compiler
> > > > see that we know what we're talking about, and we'll never ask to
> > > > write more that sizeof(ucmd.cmd) bytes during the memset() inside
> > > > copy_struct_from_user(). Seen under GCC 13:
> > > >
> > > > In function 'copy_struct_from_user',
> > > > inlined from 'iommufd_fops_ioctl' at ../drivers/iommu/iommufd/main.c:333:8:
> > > > ../include/linux/fortify-string.h:59:33: warning: '__builtin_memset' offset [57, 4294967294] is out of the bounds [0, 56] of object 'buf' with type 'union ucmd_buffer' [-Warray-bounds=]
> > > > 59 | #define __underlying_memset __builtin_memset
> > >
> > > This seems strange to me
> > >
> > > I thought the way gcc handled this was if it knew the value must be in
> > > a certain range then it would check it
> > >
> > > If it couldn't figure out any ranges it would not make a warning.
> > >
> > > So why did it decide "rest" was in that really weird range?
> >
> > It's because it got bounds-checked at the lower end (for the minimum
> > size test).
>
> Where? There is no sizeof(ucmd.ubuffer) in this code.
memset() is internally doing that via __builtin_object_size(dst + size, 1).
> There are no statically computable constants at all.
I think it's some logic that excludes a range based on ucmd.user_size
internally to the additional checks in copy_struct_from_user().
Regardless, I think the correct fix should be with
copy_struct_from_user(), so please disregard this patch. :)
--
Kees Cook
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2023-02-01 1:02 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2023-01-27 22:38 [PATCH] iommufd: Add top-level bounds check on kernel buffer size Kees Cook
2023-01-28 0:47 ` Jason Gunthorpe
2023-01-28 0:57 ` Kees Cook
2023-01-28 1:13 ` Jason Gunthorpe
2023-02-01 1:01 ` Kees Cook
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.