linux-kernel - Re: [PATCH] binfmt_elf: Take the mmap lock when walking the VMA list

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Date:   Mon, 31 Jan 2022 10:03:31 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Denys Vlasenko <vda.linux@...glemail.com>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>
Subject: Re: [PATCH] binfmt_elf: Take the mmap lock when walking the VMA list

"Matthew Wilcox (Oracle)" <willy@...radead.org> writes:

> I'm not sure if the VMA list can change under us, but dump_vma_snapshot()
> is very careful to take the mmap_lock in write mode.  We only need to
> take it in read mode here as we do not care if the size of the stack
> VMA changes underneath us.
>
> If it can be changed underneath us, this is a potential use-after-free
> for a multithreaded process which is dumping core.

The problem is not multi-threaded process so much as processes that
share their mm.

I think rather than take a lock we should be using the snapshot captured
with dump_vma_snapshot.  Otherwise we have the very real chance that the
two get out of sync.  Which would result in a non-sense core file.

Probably that means that dump_vma_snapshot needs to call get_file on
vma->vm_file store it in core_vma_metadata.

Do you think you can fix it something like that?

Eric


> Fixes: 2aa362c49c31 ("coredump: extend core dump note section to contain file names of mapped files")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  fs/binfmt_elf.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 605017eb9349..dc2318355762 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1651,6 +1651,7 @@ static int fill_files_note(struct memelfnote *note)
>  	name_base = name_curpos = ((char *)data) + names_ofs;
>  	remaining = size - names_ofs;
>  	count = 0;
> +	mmap_read_lock(mm);
>  	for (vma = mm->mmap; vma != NULL; vma = vma->vm_next) {
>  		struct file *file;
>  		const char *filename;
> @@ -1661,6 +1662,7 @@ static int fill_files_note(struct memelfnote *note)
>  		filename = file_path(file, name_curpos, remaining);
>  		if (IS_ERR(filename)) {
>  			if (PTR_ERR(filename) == -ENAMETOOLONG) {
> +				mmap_read_unlock(mm);
>  				kvfree(data);
>  				size = size * 5 / 4;
>  				goto alloc;
> @@ -1680,6 +1682,7 @@ static int fill_files_note(struct memelfnote *note)
>  		*start_end_ofs++ = vma->vm_pgoff;
>  		count++;
>  	}
> +	mmap_read_unlock(mm);
>  
>  	/* Now we know exact count of files, can store it */
>  	data[0] = count;