linux-kernel - [ 074/117] ARM: 7409/1: Do not call flush_cache_user_range with mmap

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Message-Id: <20120528031213.085639353@decadent.org.uk>
Date:	Mon, 28 May 2012 04:13:16 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Dima Zavin <dima@...roid.com>,
	Catalin Marinas <catalin.marinas@....com>,
	John Stultz <john.stultz@...aro.org>,
	Will Deacon <will.deacon@....com>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: [ 074/117] ARM: 7409/1: Do not call flush_cache_user_range with mmap_sem held

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dima Zavin <dima@...roid.com>

commit 435a7ef52db7d86e67a009b36cac1457f8972391 upstream.

We can't be holding the mmap_sem while calling flush_cache_user_range
because the flush can fault. If we fault on a user address, the
page fault handler will try to take mmap_sem again. Since both places
acquire the read lock, most of the time it succeeds. However, if another
thread tries to acquire the write lock on the mmap_sem (e.g. mmap) in
between the call to flush_cache_user_range and the fault, the down_read
in do_page_fault will deadlock.

[will: removed drop of vma parameter as already queued by rmk (7365/1)]

Acked-by: Catalin Marinas <catalin.marinas@....com>
Signed-off-by: Dima Zavin <dima@...roid.com>
Signed-off-by: John Stultz <john.stultz@...aro.org>
Signed-off-by: Will Deacon <will.deacon@....com>
Signed-off-by: Russell King <rmk+kernel@....linux.org.uk>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/arm/kernel/traps.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 55b2f3d..63d402f 100644
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -496,7 +496,9 @@ do_cache_op(unsigned long start, unsigned long end, int flags)
 		if (end > vma->vm_end)
 			end = vma->vm_end;
 
+		up_read(&mm->mmap_sem);
 		flush_cache_user_range(start, end);
+		return;
 	}
 	up_read(&mm->mmap_sem);
 }


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