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@xxxxxxxxxxxxxxxxx>
Cc: Mateusz Guzik <mguzik@xxxxxxxxxx>
CC: Cyrill Gorcunov <gorcunov@xxxxxxxxx>
Co-developed-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Signed-off-by: Michal Koutný <mkoutny@xxxxxxxx>
---
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
