00d2cb8066
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.com
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>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.