linux-kernel - [PATCH 3.16 29/63] staging: ashmem: Avoid deadlock with mmap/shrink

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Message-ID: <lsq.1578512578.165540140@decadent.org.uk>
Date:   Wed, 08 Jan 2020 19:43:27 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, Denis Kirjanov <kda@...ux-powerpc.org>,
        "Android Kernel Team" <kernel-team@...roid.com>,
        "Laura Abbott" <lauraa@...eaurora.org>,
        "Osvaldo Banuelos" <osvaldob@...eaurora.org>,
        "John Stultz" <john.stultz@...aro.org>,
        "Syed Rameez Mustafa" <rameezmustafa@...eaurora.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        "Subbaraman Narayanamurthy" <subbaram@...eaurora.org>,
        "Greg KH" <gregkh@...uxfoundation.org>,
        "Matt Wagantall" <mattw@...eaurora.org>
Subject: [PATCH 3.16 29/63] staging: ashmem: Avoid deadlock with mmap/shrink

3.16.81-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Laura Abbott <lauraa@...eaurora.org>

commit 18e77054de741ef3ed2a2489bc9bf82a318b2d5e upstream.

Both ashmem_mmap and ashmem_shrink take the ashmem_lock. It may
be possible for ashmem_mmap to invoke ashmem_shrink:

-000|mutex_lock(lock = 0x0)
-001|ashmem_shrink(?, sc = 0x0) <--- try to take ashmem_mutex again
-002|shrink_slab(shrink = 0xDA5F1CC0, nr_pages_scanned = 0, lru_pages
-002|=
-002|124)
-003|try_to_free_pages(zonelist = 0x0, ?, ?, ?)
-004|__alloc_pages_nodemask(gfp_mask = 21200, order = 1, zonelist =
-004|0xC11D0940,
-005|new_slab(s = 0xE4841E80, ?, node = -1)
-006|__slab_alloc.isra.43.constprop.50(s = 0xE4841E80, gfpflags =
-006|2148925462, ad
-007|kmem_cache_alloc(s = 0xE4841E80, gfpflags = 208)
-008|shmem_alloc_inode(?)
-009|alloc_inode(sb = 0xE480E800)
-010|new_inode_pseudo(?)
-011|new_inode(?)
-012|shmem_get_inode(sb = 0xE480E800, dir = 0x0, ?, dev = 0, flags =
-012|187)
-013|shmem_file_setup(?, ?, flags = 187)
-014|ashmem_mmap(?, vma = 0xC5D64210) <---- Acquire ashmem_mutex
-015|mmap_region(file = 0xDF8E2C00, addr = 1772974080, len = 233472,
-015|flags = 57,
-016|sys_mmap_pgoff(addr = 0, len = 230400, prot = 3, flags = 1, fd =
-016|157, pgoff
-017|ret_fast_syscall(asm)
-->|exception
-018|NUR:0x40097508(asm)
---|end of frame

Avoid this deadlock by using mutex_trylock in ashmem_shrink; if the mutex
is already held, do not attempt to shrink.

Cc: Greg KH <gregkh@...uxfoundation.org>
Cc: Android Kernel Team <kernel-team@...roid.com>
Reported-by: Matt Wagantall <mattw@...eaurora.org>
Reported-by: Syed Rameez Mustafa <rameezmustafa@...eaurora.org>
Reported-by: Osvaldo Banuelos <osvaldob@...eaurora.org>
Reported-by: Subbaraman Narayanamurthy <subbaram@...eaurora.org>
Signed-off-by: Laura Abbott <lauraa@...eaurora.org>
[jstultz: Minor commit message tweaks]
Signed-off-by: John Stultz <john.stultz@...aro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Arnd Bergmann <arnd@...db.de>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/staging/android/ashmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -434,7 +434,9 @@ ashmem_shrink_scan(struct shrinker *shri
 	if (!(sc->gfp_mask & __GFP_FS))
 		return SHRINK_STOP;
 
-	mutex_lock(&ashmem_mutex);
+	if (!mutex_trylock(&ashmem_mutex))
+		return -1;
+
 	list_for_each_entry_safe(range, next, &ashmem_lru_list, lru) {
 		loff_t start = range->pgstart * PAGE_SIZE;
 		loff_t end = (range->pgend + 1) * PAGE_SIZE;