[PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Xenia Ragiadakou]

The function snprintf() returns the number of characters that would have been
written in the buffer if the buffer size had been sufficiently large,
not counting the terminating null character.
Hence, the value returned is not guaranteed to be smaller than the buffer size.
Check the return value of snprintf to prevent leaking stack contents to the
guest by accident.

Also, for debug builds, add an assertion to ensure that the assumption made on
the size of the destination buffer still holds.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
---

Changes in v2:
- add ASSERT_UNREACHABLE()
- update commit message accordingly

 xen/common/hypfs.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/common/hypfs.c b/xen/common/hypfs.c
index 66026ad3e0..7b3377d46e 100644
--- a/xen/common/hypfs.c
+++ b/xen/common/hypfs.c
@@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct hypfs_entry_dir *template,
     unsigned int e_namelen, e_len;
 
     e_namelen = snprintf(name, sizeof(name), template->e.name, id);
-    if ( e_namelen >= sizeof(name) )
+    if ( e_namelen >= sizeof(name) ) {
+        ASSERT_UNREACHABLE();
         return -ENOBUFS;
+    }
     e_len = DIRENTRY_SIZE(e_namelen);
     direntry.e.pad = 0;
     direntry.e.type = template->e.type;
-- 
2.34.1

Re: [PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Jan Beulich]

On 04.08.2022 14:47, Xenia Ragiadakou wrote:
> Changes in v2:
> - add ASSERT_UNREACHABLE()

Hmm, this ...

> --- a/xen/common/hypfs.c
> +++ b/xen/common/hypfs.c
> @@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct hypfs_entry_dir *template,
>      unsigned int e_namelen, e_len;
>  
>      e_namelen = snprintf(name, sizeof(name), template->e.name, id);
> -    if ( e_namelen >= sizeof(name) )
> +    if ( e_namelen >= sizeof(name) ) {
> +        ASSERT_UNREACHABLE();
>          return -ENOBUFS;
> +    }

... looks to be an incremental patch on top of v1, not v2 of that
patch?

Also please correct the placement of the opening brace.

Jan

Re: [PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Xenia Ragiadakou]

Hi Jan,

On 8/4/22 16:01, Jan Beulich wrote:
> On 04.08.2022 14:47, Xenia Ragiadakou wrote:
>> Changes in v2:
>> - add ASSERT_UNREACHABLE()
> 
> Hmm, this ...
> 
>> --- a/xen/common/hypfs.c
>> +++ b/xen/common/hypfs.c
>> @@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct hypfs_entry_dir *template,
>>       unsigned int e_namelen, e_len;
>>   
>>       e_namelen = snprintf(name, sizeof(name), template->e.name, id);
>> -    if ( e_namelen >= sizeof(name) )
>> +    if ( e_namelen >= sizeof(name) ) {
>> +        ASSERT_UNREACHABLE();
>>           return -ENOBUFS;
>> +    }
> 
> ... looks to be an incremental patch on top of v1, not v2 of that
> patch?

So, here, IIUC, I have to create a patch series and add the assert in 
the second (2/2) patch? What should be the version number of the series?

> Also please correct the placement of the opening brace.

Ah, ok. Sorry.

-- 
Xenia

Re: [PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Jan Beulich]

On 04.08.2022 15:10, Xenia Ragiadakou wrote:
> On 8/4/22 16:01, Jan Beulich wrote:
>> On 04.08.2022 14:47, Xenia Ragiadakou wrote:
>>> Changes in v2:
>>> - add ASSERT_UNREACHABLE()
>>
>> Hmm, this ...
>>
>>> --- a/xen/common/hypfs.c
>>> +++ b/xen/common/hypfs.c
>>> @@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct hypfs_entry_dir *template,
>>>       unsigned int e_namelen, e_len;
>>>   
>>>       e_namelen = snprintf(name, sizeof(name), template->e.name, id);
>>> -    if ( e_namelen >= sizeof(name) )
>>> +    if ( e_namelen >= sizeof(name) ) {
>>> +        ASSERT_UNREACHABLE();
>>>           return -ENOBUFS;
>>> +    }
>>
>> ... looks to be an incremental patch on top of v1, not v2 of that
>> patch?
> 
> So, here, IIUC, I have to create a patch series and add the assert in 
> the second (2/2) patch? What should be the version number of the series?

No, why? Simply fold this change into the earlier one, and call the
result v3.

Jan

Re: [PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Xenia Ragiadakou]

On 8/4/22 16:13, Jan Beulich wrote:
> On 04.08.2022 15:10, Xenia Ragiadakou wrote:
>> On 8/4/22 16:01, Jan Beulich wrote:
>>> On 04.08.2022 14:47, Xenia Ragiadakou wrote:
>>>> Changes in v2:
>>>> - add ASSERT_UNREACHABLE()
>>>
>>> Hmm, this ...
>>>
>>>> --- a/xen/common/hypfs.c
>>>> +++ b/xen/common/hypfs.c
>>>> @@ -377,8 +377,10 @@ int hypfs_read_dyndir_id_entry(const struct hypfs_entry_dir *template,
>>>>        unsigned int e_namelen, e_len;
>>>>    
>>>>        e_namelen = snprintf(name, sizeof(name), template->e.name, id);
>>>> -    if ( e_namelen >= sizeof(name) )
>>>> +    if ( e_namelen >= sizeof(name) ) {
>>>> +        ASSERT_UNREACHABLE();
>>>>            return -ENOBUFS;
>>>> +    }
>>>
>>> ... looks to be an incremental patch on top of v1, not v2 of that
>>> patch?
>>
>> So, here, IIUC, I have to create a patch series and add the assert in
>> the second (2/2) patch? What should be the version number of the series?
> 
> No, why? Simply fold this change into the earlier one, and call the
> result v3.

Okkk, I just realized what I have done.

-- 
Xenia

Re: [PATCH v2] xen/hypfs: check the return value of snprintf to avoid leaking stack accidently

[Juergen Gross]

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On 04.08.22 14:47, Xenia Ragiadakou wrote:
> The function snprintf() returns the number of characters that would have been
> written in the buffer if the buffer size had been sufficiently large,
> not counting the terminating null character.
> Hence, the value returned is not guaranteed to be smaller than the buffer size.
> Check the return value of snprintf to prevent leaking stack contents to the
> guest by accident.
> 
> Also, for debug builds, add an assertion to ensure that the assumption made on
> the size of the destination buffer still holds.
> 
> Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>

Reviewed-by: Juergen Gross <jgross@suse.com>


Juergen

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