[PATCH AUTOSEL 5.1 140/141] mm: use down_read_killable for locking mmap_sem in access_remote_vm

Sasha Levin
2019-07-19T04:07:28Z

From: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>

[ Upstream commit 1e426fe28261b03f297992e89da3320b42816f4e ]

This function is used by ptrace and proc files like /proc/pid/cmdline and
/proc/pid/environ.

Access_remote_vm never returns error codes, all errors are ignored and
only size of successfully read data is returned. So, if current task was
killed we'll simply return 0 (bytes read).

Mmap_sem could be locked for a long time or forever if something goes
wrong. Using a killable lock permits cleanup of stuck tasks and
simplifies investigation.

Link: http://lkml.kernel.org/r/156007494202.3335.16782303099589302087.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Kirill Tkhai <ktkhai@virtuozzo.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Roman Gushchin <guro@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/memory.c | 4 +++-
mm/nommu.c | 3 ++-
2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index ab650c21bccd..57402801ab09 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4260,7 +4260,9 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
void *old_buf = buf;
int write = gup_flags & FOLL_WRITE;

- down_read(&mm->mmap_sem);
+ if (down_read_killable(&mm->mmap_sem))
+ return 0;
+
/* ignore errors, just check how much was successfully transferred */
while (len) {
int bytes, ret, offset;
diff --git a/mm/nommu.c b/mm/nommu.c
index 749276beb109..1bd91ceadc82 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1777,7 +1777,8 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
struct vm_area_struct *vma;
int write = gup_flags & FOLL_WRITE;

- down_read(&mm->mmap_sem);
+ if (down_read_killable(&mm->mmap_sem))
+ return 0;

/* the access must start within one of the target process's mappings */
vma = find_vma(mm, addr);
--
2.20.1