mm: shmem: NULL ptr deref in shmem_fault

[Sasha Levin <sasha.levin@oracle.com>]

Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel I've stumbled on the following spew.

It seems that in this case, 'inode->i_mapping' was NULL, and the deref happened
when we tried to get it's flags in mapping_gfp_mask().

[ 4431.615828] BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
[ 4431.617708] IP: shmem_fault (mm/shmem.c:2960 mm/shmem.c:1236)
[ 4431.621711] PGD 1d7fb5067 PUD 1d7fb4067 PMD 0
[ 4431.621945] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 4431.621945] Dumping ftrace buffer:
[ 4431.621945]    (ftrace buffer empty)
[ 4431.621945] Modules linked in:
[ 4431.621945] CPU: 1 PID: 20571 Comm: trinity-c61 Not tainted 3.15.0-rc5-next-20140512-sasha-00019-ga20bc00-dirty #456
[ 4431.621945] task: ffff8803333e0000 ti: ffff88032b562000 task.ti: ffff88032b562000
[ 4431.621945] RIP: shmem_fault (mm/shmem.c:2960 mm/shmem.c:1236)
[ 4431.621945] RSP: 0018:ffff88032b5639b8  EFLAGS: 00010296
[ 4431.621945] RAX: ffff88005daab100 RBX: ffff88005db19e00 RCX: 0000000000000001
[ 4431.621945] RDX: 0000000000000001 RSI: ffff88032b5639f8 RDI: 0000000000000000
[ 4431.621945] RBP: ffff88032b5639d8 R08: ffff88032b563a70 R09: ffff88032b5639c4
[ 4431.621945] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8801caad2ed0
[ 4431.621945] R13: ffff8801ca27c7e8 R14: 0000000000000000 R15: ffff88005db19e00
[ 4431.621945] FS:  00007f8c3b4ef700(0000) GS:ffff88006ec00000(0000) knlGS:0000000000000000
[ 4431.621945] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 4431.621945] CR2: 0000000000000030 CR3: 00000001d7fb6000 CR4: 00000000000006a0
[ 4431.621945] DR0: 00000000006df000 DR1: 0000000000000000 DR2: 0000000000000000
[ 4431.621945] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
[ 4431.621945] Stack:
[ 4431.621945]  ffffffff8a2bb583 0000020000000286 ffff88032b563a18 ffff88032b563a70
[ 4431.621945]  ffff88032b563a38 ffffffff8a2b83d9 ffff8801e71f8338 00000000ffffffff
[ 4431.621945]  ffff880300000000 0000000000000001 00007f8c3b4fd000 0000000000000000
[ 4431.621945] Call Trace:
[ 4431.621945] ? do_read_fault.isra.40 (mm/memory.c:2882)
[ 4431.621945] __do_fault (mm/memory.c:2703)
[ 4431.621945] ? _raw_spin_unlock (arch/x86/include/asm/preempt.h:98 include/linux/spinlock_api_smp.h:152 kernel/locking/spinlock.c:183)
[ 4431.621945] do_read_fault.isra.40 (mm/memory.c:2883)
[ 4431.621945] ? get_parent_ip (kernel/sched/core.c:2519)
[ 4431.621945] ? get_parent_ip (kernel/sched/core.c:2519)
[ 4431.621945] __handle_mm_fault (mm/memory.c:3021 mm/memory.c:3182 mm/memory.c:3306)
[ 4431.621945] ? __const_udelay (arch/x86/lib/delay.c:126)
[ 4431.621945] ? __rcu_read_unlock (kernel/rcu/update.c:97)
[ 4431.621945] handle_mm_fault (mm/memory.c:3329)
[ 4431.621945] __get_user_pages (mm/gup.c:281 mm/gup.c:466)
[ 4431.621945] ? preempt_count_sub (kernel/sched/core.c:2575)
[ 4431.621945] get_user_pages (mm/gup.c:632)
[ 4431.621945] get_user_pages_fast (arch/x86/mm/gup.c:394)
[ 4431.621945] vmsplice_to_pipe (fs/splice.c:1487 fs/splice.c:1607)
[ 4431.621945] ? page_cache_pipe_buf_release (fs/splice.c:267)
[ 4431.621945] ? kvm_clock_read (arch/x86/include/asm/preempt.h:90 arch/x86/kernel/kvmclock.c:86)
[ 4431.621945] ? sched_clock (arch/x86/include/asm/paravirt.h:192 arch/x86/kernel/tsc.c:305)
[ 4431.621945] ? sched_clock_local (kernel/sched/clock.c:214)
[ 4431.621945] ? vtime_account_user (kernel/sched/cputime.c:687)
[ 4431.621945] ? debug_smp_processor_id (lib/smp_processor_id.c:57)
[ 4431.621945] ? put_lock_stats.isra.12 (arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
[ 4431.621945] ? vtime_account_user (kernel/sched/cputime.c:687)
[ 4431.621945] ? get_parent_ip (kernel/sched/core.c:2519)
[ 4431.621945] ? get_parent_ip (kernel/sched/core.c:2519)
[ 4431.621945] ? __fget_light (include/linux/rcupdate.h:428 include/linux/fdtable.h:80 fs/file.c:684)
[ 4431.621945] SyS_vmsplice (fs/splice.c:1650 fs/splice.c:1636)
[ 4431.621945] tracesys (arch/x86/kernel/entry_64.S:746)
[ 4431.621945] Code: 66 66 66 90 55 b9 01 00 00 00 48 89 e5 53 48 89 fb 4c 8d 4d ec 48 83 ec 18 c7 45 ec 00 02 00 00 48 8b 87 a0 00 00 00 48 8b 78 20 <48> 8b 57 30 4c 8b 82 48 01 00 00 48 8d 56 18 48 8b 76 08 41 81
[ 4431.621945] RIP shmem_fault (mm/shmem.c:2960 mm/shmem.c:1236)
[ 4431.621945]  RSP <ffff88032b5639b8>
[ 4431.621945] CR2: 0000000000000030


Thanks,
Sasha

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