linux-kernel - Re: [PATCH] buildid: validate page-backed file before parsing build ID

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Message-ID: <95f3dae0-fc8c-43a8-aa37-8393e66cc75b@kernel.org>
Date: Sun, 11 Jan 2026 12:32:13 +0100
From: "David Hildenbrand (Red Hat)" <david@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Jinchao Wang <wangjinchao600@...il.com>, Song Liu <song@...nel.org>,
 Jiri Olsa <jolsa@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
 syzbot+e008db2ac01e282550ee@...kaller.appspotmail.com,
 Axel Rasmussen <axelrasmussen@...gle.com>,
 Johannes Weiner <hannes@...xchg.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Michal Hocko <mhocko@...nel.org>, Qi Zheng <zhengqi.arch@...edance.com>,
 Shakeel Butt <shakeel.butt@...ux.dev>, Wei Xu <weixugc@...gle.com>,
 Yuanchu Xie <yuanchu@...gle.com>, Omar Sandoval <osandov@...com>,
 Deepanshu Kartikey <kartikey406@...il.com>
Subject: Re: [PATCH] buildid: validate page-backed file before parsing build
 ID

On 1/10/26 00:43, Andrii Nakryiko wrote:
> On Tue, Jan 6, 2026 at 11:16 AM David Hildenbrand (Red Hat)
> <david@...nel.org> wrote:
>>
>> On 1/5/26 23:52, Andrii Nakryiko wrote:
>>> On Tue, Dec 30, 2025 at 2:11 PM David Hildenbrand (Red Hat)
>>> <david@...nel.org> wrote:
>>>>
>>>> On 12/23/25 18:29, Andrew Morton wrote:
>>>>> On Tue, 23 Dec 2025 18:32:07 +0800 Jinchao Wang <wangjinchao600@...il.com> wrote:
>>>>>
>>>>>> __build_id_parse() only works on page-backed storage.  Its helper paths
>>>>>> eventually call mapping->a_ops->read_folio(), so explicitly reject VMAs
>>>>>> that do not map a regular file or lack valid address_space operations.
>>>>>>
>>>>>> Reported-by: syzbot+e008db2ac01e282550ee@...kaller.appspotmail.com
>>>>>> Signed-off-by: Jinchao Wang <wangjinchao600@...il.com>
>>>>>>
>>>>>> ...
>>>>>>
>>>>>> --- a/lib/buildid.c
>>>>>> +++ b/lib/buildid.c
>>>>>> @@ -280,7 +280,10 @@ static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
>>>>>>        int ret;
>>>>>>
>>>>>>        /* only works for page backed storage  */
>>>>>> -    if (!vma->vm_file)
>>>>>> +    if (!vma->vm_file ||
>>>>>> +        !S_ISREG(file_inode(vma->vm_file)->i_mode) ||
>>>>>> +        !vma->vm_file->f_mapping->a_ops ||
>>>>>> +        !vma->vm_file->f_mapping->a_ops->read_folio)
>>>>>>                return -EINVAL;
>>>>
>>>> Just wondering, we are fine with MAP_PRIVATE files, right? I guess it's
>>>> not about the actual content in the VMA (which might be different for a
>>>> MAP_PRIVATE VMA), but only about the content of the mapped file.
>>>
>>> Yep, this code is fetching contents of a file that backs given VMA.
>>
>> Good!
>>
>>>
>>>>
>>>>
>>>> LGTM, although I wonder whether some of these these checks should be
>>>> exposed as part of the read_cache_folio()/do_read_cache_folio() API.
>>>>
>>>> Like, having a helper function that tells us whether we can use
>>>> do_read_cache_folio() against a given mapping+file.
>>>
>>> I agree, this seems to be leaking a lot of internal mm details into
>>> higher-level caller (__build_id_parse). Right now we try to fetch
>>> folio with filemap_get_folio() and if that succeeds, then we do
>>> read_cache_folio. Would it be possible for filemap_get_folio() to
>>> return error if the folio cannot be read using read_cache_folio()? Or
>>> maybe have a variant of filemap_get_folio() that would have this
>>> semantic?
>>
>> Good question. But really, for files that always have everything in the pagecache,
>> there would not be a problem, right? I'm thinking about hugetlb, for example.
>>
>> There, we never expect to fallback to do_read_cache_folio().
>>
>> So maybe we could just teach do_read_cache_folio() to fail properly?
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index ebd75684cb0a7..3f81b8481af4c 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -4051,8 +4051,11 @@ static struct folio *do_read_cache_folio(struct address_space *mapping,
>>           struct folio *folio;
>>           int err;
>>
>> -       if (!filler)
>> +       if (!filler) {
>> +               if (!mapping->a_ops || !mapping->a_ops->read_folio)
>> +                       return ERR_PTR(-EINVAL);
>>                   filler = mapping->a_ops->read_folio;
>> +       }
>>    repeat:
>>           folio = filemap_get_folio(mapping, index);
>>           if (IS_ERR(folio)) {
>>
>> Then __build_id_parse() would only check for the existence of vma->vm_file and maybe
>> the !S_ISREG(file_inode(vma->vm_file)->i_mode).
>>
> 
> That would be great. But something like this was proposed earlier and
> Matthew didn't particularly like this approach ([0]).
> 
>    [0] https://lore.kernel.org/all/aReUv1kVACh3UKv-@casper.infradead.org/

Well, but on the higher level we don't know whether we have to even call 
into read_cache_folio().

Again, hugetlb. Might be worth reproducing with hugetlb/shmem, and 
making sure it keeps working even with your changes.

-- 
Cheers

David