This is a note to let you know that I've just added the patch titled
proc: nommu: /proc/<pid>/maps: release mmap read lock
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
proc-nommu-proc-pid-maps-release-mmap-read-lock.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 578d7699e5c2add8c2e9549d9d75dfb56c460cb3 Mon Sep 17 00:00:00 2001
From: Ben Wolsieffer <Ben.Wolsieffer@xxxxxxxxxxx>
Date: Thu, 14 Sep 2023 12:30:20 -0400
Subject: proc: nommu: /proc/<pid>/maps: release mmap read lock
From: Ben Wolsieffer <Ben.Wolsieffer@xxxxxxxxxxx>
commit 578d7699e5c2add8c2e9549d9d75dfb56c460cb3 upstream.
The no-MMU implementation of /proc/<pid>/map doesn't normally release
the mmap read lock, because it uses !IS_ERR_OR_NULL(_vml) to determine
whether to release the lock. Since _vml is NULL when the end of the
mappings is reached, the lock is not released.
Reading /proc/1/maps twice doesn't cause a hang because it only
takes the read lock, which can be taken multiple times and therefore
doesn't show any problem if the lock isn't released. Instead, you need
to perform some operation that attempts to take the write lock after
reading /proc/<pid>/maps. To actually reproduce the bug, compile the
following code as 'proc_maps_bug':
#include <stdio.h>
#include <unistd.h>
#include <sys/mman.h>
int main(int argc, char *argv[]) {
void *buf;
sleep(1);
buf = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
puts("mmap returned");
return 0;
}
Then, run:
./proc_maps_bug &; cat /proc/$!/maps; fg
Without this patch, mmap() will hang and the command will never
complete.
This code was incorrectly adapted from the MMU implementation, which at
the time released the lock in m_next() before returning the last entry.
The MMU implementation has diverged further from the no-MMU version since
then, so this patch brings their locking and error handling into sync,
fixing the bug and hopefully avoiding similar issues in the future.
Link: https://lkml.kernel.org/r/20230914163019.4050530-2-ben.wolsieffer@xxxxxxxxxxx
Fixes: 47fecca15c09 ("fs/proc/task_nommu.c: don't use priv->task->mm")
Signed-off-by: Ben Wolsieffer <ben.wolsieffer@xxxxxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Giulio Benetti <giulio.benetti@xxxxxxxxxxxxxxxxxxxxxx>
Cc: Greg Ungerer <gerg@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/proc/task_nommu.c | 27 +++++++++++++++------------
1 file changed, 15 insertions(+), 12 deletions(-)
--- a/fs/proc/task_nommu.c
+++ b/fs/proc/task_nommu.c
@@ -208,11 +208,16 @@ static void *m_start(struct seq_file *m,
return ERR_PTR(-ESRCH);
mm = priv->mm;
- if (!mm || !mmget_not_zero(mm))
+ if (!mm || !mmget_not_zero(mm)) {
+ put_task_struct(priv->task);
+ priv->task = NULL;
return NULL;
+ }
if (mmap_read_lock_killable(mm)) {
mmput(mm);
+ put_task_struct(priv->task);
+ priv->task = NULL;
return ERR_PTR(-EINTR);
}
@@ -221,23 +226,21 @@ static void *m_start(struct seq_file *m,
if (n-- == 0)
return p;
- mmap_read_unlock(mm);
- mmput(mm);
return NULL;
}
-static void m_stop(struct seq_file *m, void *_vml)
+static void m_stop(struct seq_file *m, void *v)
{
struct proc_maps_private *priv = m->private;
+ struct mm_struct *mm = priv->mm;
- if (!IS_ERR_OR_NULL(_vml)) {
- mmap_read_unlock(priv->mm);
- mmput(priv->mm);
- }
- if (priv->task) {
- put_task_struct(priv->task);
- priv->task = NULL;
- }
+ if (!priv->task)
+ return;
+
+ mmap_read_unlock(mm);
+ mmput(mm);
+ put_task_struct(priv->task);
+ priv->task = NULL;
}
static void *m_next(struct seq_file *m, void *_p, loff_t *pos)
Patches currently in stable-queue which might be from Ben.Wolsieffer@xxxxxxxxxxx are
queue-5.15/proc-nommu-proc-pid-maps-release-mmap-read-lock.patch
