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    SubjectRe: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr
    On Mon, Jun 03, 2019 at 06:55:04PM +0200, Andrey Konovalov wrote:
    > 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>

    Reviewed-by: Kees Cook <keescook@chromium.org>

    -Kees

    > ---
    > 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();
    > --
    > 2.22.0.rc1.311.g5d7573a151-goog
    >

    --
    Kees Cook

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