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

[Sasha Levin]

From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>

[ 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@xxxxxxxxxxxxxx>
Reviewed-by: Michal Koutnà <mkoutny@xxxxxxxx>
Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Cc: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Cc: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Roman Gushchin <guro@xxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 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