[PATCH] binfmt_elf: Take the mmap lock when walking the VMA list

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

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.

Fixes: 2aa362c49c31 ("coredump: extend core dump note section to contain file names of mapped files")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@Oracle.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;
-- 
2.34.1