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    SubjectRe: [PATCH v18 07/15] fs/namespace: untag user pointers in copy_mount_options
    On Mon, Jun 24, 2019 at 04:32:52PM +0200, Andrey Konovalov wrote:
    > This patch is a part of a series that extends kernel ABI to allow to pass
    > tagged user pointers (with the top byte set to something else other than
    > 0x00) as syscall arguments.
    >
    > In copy_mount_options a user address is being subtracted from TASK_SIZE.
    > If the address is lower than TASK_SIZE, the size is calculated to not
    > allow the exact_copy_from_user() call to cross TASK_SIZE boundary.
    > However if the address is tagged, then the size will be calculated
    > incorrectly.
    >
    > Untag the address before subtracting.
    >
    > Reviewed-by: Khalid Aziz <khalid.aziz@oracle.com>
    > Reviewed-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
    > Reviewed-by: Kees Cook <keescook@chromium.org>
    > Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
    > ---
    > fs/namespace.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/fs/namespace.c b/fs/namespace.c
    > index 7660c2749c96..ec78f7223917 100644
    > --- a/fs/namespace.c
    > +++ b/fs/namespace.c
    > @@ -2994,7 +2994,7 @@ void *copy_mount_options(const void __user * data)
    > * the remainder of the page.
    > */
    > /* copy_from_user cannot cross TASK_SIZE ! */
    > - size = TASK_SIZE - (unsigned long)data;
    > + size = TASK_SIZE - (unsigned long)untagged_addr(data);
    > if (size > PAGE_SIZE)
    > size = PAGE_SIZE;

    I think this patch needs an ack from Al Viro (cc'ed).

    --
    Catalin

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