5.10-stable review patch. If anyone has any objections, please let me know.
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From: Ben Wolsieffer Ben.Wolsieffer@hefring.com
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@hefring.co... Fixes: 47fecca15c09 ("fs/proc/task_nommu.c: don't use priv->task->mm") Signed-off-by: Ben Wolsieffer ben.wolsieffer@hefring.com Acked-by: Oleg Nesterov oleg@redhat.com Cc: Giulio Benetti giulio.benetti@benettiengineering.com Cc: Greg Ungerer gerg@uclinux.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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)