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    Subject[PATCH v14 02/17] arm64: untag user pointers in access_ok and __uaccess_mask_ptr
    From
    This patch is a part of a series that extends arm64 kernel ABI to allow to
    pass tagged user pointers (with the top byte set to something else other
    than 0x00) as syscall arguments.

    copy_from_user (and a few other similar functions) are used to copy data
    from user memory into the kernel memory or vice versa. Since a user can
    provided a tagged pointer to one of the syscalls that use copy_from_user,
    we need to correctly handle such pointers.

    Do this by untagging user pointers in access_ok and in __uaccess_mask_ptr,
    before performing access validity checks.

    Note, that this patch only temporarily untags the pointers to perform the
    checks, but then passes them as is into the kernel internals.

    Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
    ---
    arch/arm64/include/asm/uaccess.h | 10 ++++++----
    1 file changed, 6 insertions(+), 4 deletions(-)

    diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
    index e5d5f31c6d36..9164ecb5feca 100644
    --- a/arch/arm64/include/asm/uaccess.h
    +++ b/arch/arm64/include/asm/uaccess.h
    @@ -94,7 +94,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
    return ret;
    }

    -#define access_ok(addr, size) __range_ok(addr, size)
    +#define access_ok(addr, size) __range_ok(untagged_addr(addr), size)
    #define user_addr_max get_fs

    #define _ASM_EXTABLE(from, to) \
    @@ -226,7 +226,8 @@ static inline void uaccess_enable_not_uao(void)

    /*
    * Sanitise a uaccess pointer such that it becomes NULL if above the
    - * current addr_limit.
    + * current addr_limit. In case the pointer is tagged (has the top byte set),
    + * untag the pointer before checking.
    */
    #define uaccess_mask_ptr(ptr) (__typeof__(ptr))__uaccess_mask_ptr(ptr)
    static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
    @@ -234,10 +235,11 @@ static inline void __user *__uaccess_mask_ptr(const void __user *ptr)
    void __user *safe_ptr;

    asm volatile(
    - " bics xzr, %1, %2\n"
    + " bics xzr, %3, %2\n"
    " csel %0, %1, xzr, eq\n"
    : "=&r" (safe_ptr)
    - : "r" (ptr), "r" (current_thread_info()->addr_limit)
    + : "r" (ptr), "r" (current_thread_info()->addr_limit),
    + "r" (untagged_addr(ptr))
    : "cc");

    csdb();
    --
    2.21.0.593.g511ec345e18-goog
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