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    Subject[PATCH 5.4 30/36] proc: use untagged_addr() for pagemap_read addresses
    Date
    From: Miles Chen <miles.chen@mediatek.com>

    commit 40d6366e9d86d9a67b5642040e76082fdb5bdcf9 upstream.

    When we try to visit the pagemap of a tagged userspace pointer, we find
    that the start_vaddr is not correct because of the tag.
    To fix it, we should untag the userspace pointers in pagemap_read().

    I tested with 5.10-rc4 and the issue remains.

    Explanation from Catalin in [1]:

    "Arguably, that's a user-space bug since tagged file offsets were never
    supported. In this case it's not even a tag at bit 56 as per the arm64
    tagged address ABI but rather down to bit 47. You could say that the
    problem is caused by the C library (malloc()) or whoever created the
    tagged vaddr and passed it to this function. It's not a kernel
    regression as we've never supported it.

    Now, pagemap is a special case where the offset is usually not
    generated as a classic file offset but rather derived by shifting a
    user virtual address. I guess we can make a concession for pagemap
    (only) and allow such offset with the tag at bit (56 - PAGE_SHIFT + 3)"

    My test code is based on [2]:

    A userspace pointer which has been tagged by 0xb4: 0xb400007662f541c8

    userspace program:

    uint64 OsLayer::VirtualToPhysical(void *vaddr) {
    uint64 frame, paddr, pfnmask, pagemask;
    int pagesize = sysconf(_SC_PAGESIZE);
    off64_t off = ((uintptr_t)vaddr) / pagesize * 8; // off = 0xb400007662f541c8 / pagesize * 8 = 0x5a00003b317aa0
    int fd = open(kPagemapPath, O_RDONLY);
    ...

    if (lseek64(fd, off, SEEK_SET) != off || read(fd, &frame, 8) != 8) {
    int err = errno;
    string errtxt = ErrorString(err);
    if (fd >= 0)
    close(fd);
    return 0;
    }
    ...
    }

    kernel fs/proc/task_mmu.c:

    static ssize_t pagemap_read(struct file *file, char __user *buf,
    size_t count, loff_t *ppos)
    {
    ...
    src = *ppos;
    svpfn = src / PM_ENTRY_BYTES; // svpfn == 0xb400007662f54
    start_vaddr = svpfn << PAGE_SHIFT; // start_vaddr == 0xb400007662f54000
    end_vaddr = mm->task_size;

    /* watch out for wraparound */
    // svpfn == 0xb400007662f54
    // (mm->task_size >> PAGE) == 0x8000000
    if (svpfn > mm->task_size >> PAGE_SHIFT) // the condition is true because of the tag 0xb4
    start_vaddr = end_vaddr;

    ret = 0;
    while (count && (start_vaddr < end_vaddr)) { // we cannot visit correct entry because start_vaddr is set to end_vaddr
    int len;
    unsigned long end;
    ...
    }
    ...
    }

    [1] https://lore.kernel.org/patchwork/patch/1343258/
    [2] https://github.com/stressapptest/stressapptest/blob/master/src/os.cc#L158

    Link: https://lkml.kernel.org/r/20201204024347.8295-1-miles.chen@mediatek.com
    Signed-off-by: Miles Chen <miles.chen@mediatek.com>
    Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Alexey Dobriyan <adobriyan@gmail.com>
    Cc: Andrey Konovalov <andreyknvl@google.com>
    Cc: Alexander Potapenko <glider@google.com>
    Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
    Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
    Cc: Catalin Marinas <catalin.marinas@arm.com>
    Cc: Dmitry Vyukov <dvyukov@google.com>
    Cc: Marco Elver <elver@google.com>
    Cc: Will Deacon <will@kernel.org>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Cc: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>
    Cc: <stable@vger.kernel.org> [5.4-]
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    fs/proc/task_mmu.c | 8 ++++++--
    1 file changed, 6 insertions(+), 2 deletions(-)

    --- a/fs/proc/task_mmu.c
    +++ b/fs/proc/task_mmu.c
    @@ -1567,11 +1567,15 @@ static ssize_t pagemap_read(struct file

    src = *ppos;
    svpfn = src / PM_ENTRY_BYTES;
    - start_vaddr = svpfn << PAGE_SHIFT;
    end_vaddr = mm->task_size;

    /* watch out for wraparound */
    - if (svpfn > mm->task_size >> PAGE_SHIFT)
    + start_vaddr = end_vaddr;
    + if (svpfn <= (ULONG_MAX >> PAGE_SHIFT))
    + start_vaddr = untagged_addr(svpfn << PAGE_SHIFT);
    +
    + /* Ensure the address is inside the task */
    + if (start_vaddr > mm->task_size)
    start_vaddr = end_vaddr;

    /*

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