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prctl_set_mm: downgrade mmap_sem to read lock

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The commit a3b609ef9f ("proc read mm's {arg,env}_{start,end} with mmap
semaphore taken.") added synchronization of reading argument/environment
boundaries under mmap_sem.  Later commit 88aa7cc688 ("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).

[[email protected]: coding style fixes]
Link: http://lkml.kernel.org/r/[email protected]
Fixes: 88aa7cc688 ("mm: introduce arg_lock to protect arg_start|end and env_start|end in mm_struct")
Signed-off-by: Michal Koutný <[email protected]>
Signed-off-by: Laurent Dufour <[email protected]>
Co-developed-by: Laurent Dufour <[email protected]>
Reviewed-by: Cyrill Gorcunov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Mateusz Guzik <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Konstantin Khlebnikov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Michal Koutný 2019-05-31 22:30:19 -07:00 • committed by Linus Torvalds
commit bc81426f5b
2 changed files with 11 additions and 4 deletions
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4
mm/util.c
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@ -718,12 +718,12 @@ 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;