[PATCH] mm: use down_read_killable for locking mmap_sem in access_remote_vm

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

This function is used by ptrace and proc files like /proc/pid/cmdline and
/proc/pid/environ. Return 0 (bytes read) if current task is killed.

Mmap_sem could be locked for a long time or forever if something wrong.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 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 96f1d473c89a..2e6846d09023 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4348,7 +4348,9 @@ 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 b492fd1fcf9f..cad8fb34088f 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1791,7 +1791,8 @@ 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);