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    Subject[PATCH] proc: pagemap: Hold mmap_sem during page walk
    Date
    If the mmap_sem is not held while we walk_page_range(), then
    it is possible for find_vma() to race with a remove_vma_list()
    caused by do_munmap() (or others).

    Unable to handle kernel paging request at virtual address 6b6b6b5b
    Internal error: Oops: 5 [#1] PREEMPT
    CPU: 0 Not tainted (2.6.32.9-27154-ge3e6e27 #1)
    PC is at find_vma+0x40/0x7c
    LR is at walk_page_range+0x70/0x230
    pc : [<c00aa3ac>] lr : [<c00b298c>] psr: 20000013
    sp : c6aa9eb8 ip : 6b6b6b53 fp : c6a58f60
    r10: c7e1d1b8 r9 : 0001bca0 r8 : 47000000
    r7 : c6aa9f80 r6 : c6aa8000 r5 : 46fbd000 r4 : 6b6b6b6b
    r3 : c7ca4820 r2 : 6b6b6b6b r1 : 46fbd000 r0 : c70e3e40
    Flags: nzCv IRQs on FIQs on Mode SVC_32 ISA ARM Segment user
    Control: 10c5787d Table: 26574019 DAC: 00000015

    [<c00aa3ac>] (find_vma+0x40/0x7c) from [<c00b298c>] (walk_page_range+0x70/0x230)
    [<c00b298c>] (walk_page_range+0x70/0x230) from [<c00f5d3c>] (pagemap_read+0x1a4/0x278)
    [<c00f5d3c>] (pagemap_read+0x1a4/0x278) from [<c00bac40>] (vfs_read+0xa8/0x150)
    [<c00bac40>] (vfs_read+0xa8/0x150) from [<c00bad94>] (sys_read+0x3c/0x68)
    [<c00bad94>] (sys_read+0x3c/0x68) from [<c0026f00>] (ret_fast_syscall+0x0/0x2c)
    Code: 98bd8010 e5932004 e3a00000 ea000008 (e5124010)

    Signed-off-by: San Mehat <san@google.com>
    Cc: Brian Swetland <swetland@google.com>
    Cc: Matt Mackall <mpm@selenic.com>
    Cc: Dave Hansen <haveblue@us.ibm.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    ---
    fs/proc/task_mmu.c | 3 +--
    1 files changed, 1 insertions(+), 2 deletions(-)

    diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
    index 2a1bef9..3f300c1 100644
    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -726,8 +726,6 @@ static ssize_t pagemap_read(struct file *file, char __user *buf,
    down_read(&current->mm->mmap_sem);
    ret = get_user_pages(current, current->mm, uaddr, pagecount,
    1, 0, pages, NULL);
    - up_read(&current->mm->mmap_sem);
    -
    if (ret < 0)
    goto out_free;

    @@ -776,6 +774,7 @@ out_pages:
    page_cache_release(page);
    }
    out_free:
    + up_read(&current->mm->mmap_sem);
    kfree(pages);
    out_mm:
    mmput(mm);
    --
    1.7.0.1


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