[PATCH v3 2/2] prctl_set_mm: downgrade mmap_sem to read lock

["Michal Koutný" <mkoutny@suse.com>]

The commit a3b609ef9f8b ("proc read mm's {arg,env}_{start,end} with mmap
semaphore taken.") added synchronization of reading argument/environment
boundaries under mmap_sem. Later commit 88aa7cc688d4 ("mm: introduce
arg_lock to protect arg_start|end and env_start|end in mm_struct")
avoided the coarse use of mmap_sem in similar situations. But there
still remained two places that (mis)use mmap_sem.

get_cmdline should also use arg_lock instead of mmap_sem when it reads the
boundaries.

The second place that should use arg_lock is in prctl_set_mm. By
protecting the boundaries fields with the arg_lock, we can downgrade
mmap_sem to reader lock (analogous to what we already do in
prctl_set_mm_map).

v2: call find_vma without arg_lock held
v3: squashed get_cmdline arg_lock patch

Fixes: 88aa7cc688d4 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Cc: Mateusz Guzik <mguzik@redhat.com>
CC: Cyrill Gorcunov <gorcunov@gmail.com>
Co-developed-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/sys.c | 10 ++++++++--
 mm/util.c    |  4 ++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index 5e0a5edf47f8..14be57840511 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2122,9 +2122,14 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = -EINVAL;
 
-	down_write(&mm->mmap_sem);
+	/*
+	 * arg_lock protects concurent updates of arg boundaries, we need mmap_sem for
+	 * a) concurrent sys_brk, b) finding VMA for addr validation.
+	 */
+	down_read(&mm->mmap_sem);
 	vma = find_vma(mm, addr);
 
+	spin_lock(&mm->arg_lock);
 	prctl_map.start_code	= mm->start_code;
 	prctl_map.end_code	= mm->end_code;
 	prctl_map.start_data	= mm->start_data;
@@ -2212,7 +2217,8 @@ static int prctl_set_mm(int opt, unsigned long addr,
 
 	error = 0;
 out:
-	up_write(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
+	up_read(&mm->mmap_sem);
 	return error;
 }
 
diff --git a/mm/util.c b/mm/util.c
index 43a2984bccaa..5cf0e84a0823 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -758,12 +758,12 @@ int get_cmdline(struct task_struct *task, char *buffer, int buflen)
 	if (!mm->arg_end)
 		goto out_mm;	/* Shh! No looking before we're done */
 
-	down_read(&mm->mmap_sem);
+	spin_lock(&mm->arg_lock);
 	arg_start = mm->arg_start;
 	arg_end = mm->arg_end;
 	env_start = mm->env_start;
 	env_end = mm->env_end;
-	up_read(&mm->mmap_sem);
+	spin_unlock(&mm->arg_lock);
 
 	len = arg_end - arg_start;
 
-- 
2.16.4