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    Subject[PATCH] x86, x86_64: Fix checks for userspace address limit
    Date
    hi,
    there seems to be bug in the _copy_to_user and _copy_from_user
    functions, not allowing access to the last user page.

    Also I tried to decipher the inline assembly in __range_not_ok,
    and it seems to work properly, but the macro comment seems to
    be misleading.

    wbr,
    jirka

    ---
    As shown in BZ 30352 (https://bugzilla.kernel.org/show_bug.cgi?id=30352)
    there's an issue with reading last allowed page on x86_64.

    The _copy_to_user and _copy_from_user functions use following
    check for address limit:

    if (buf + size >= limit)
    fail

    while it should be:

    if (buf + size > limit)
    fail

    That's because the size represents the number of bytes being
    read/write from/to buf address AND including the buf address.
    So the copy function will actually never touch the limit
    address even if "buf + size == limit".

    Following program fails to use the last page as buffer
    due to the wrong limit check.

    ---
    #include <sys/mman.h>
    #include <sys/socket.h>
    #include <assert.h>

    #define PAGE_SIZE (4096)
    #define LAST_PAGE ((void*)(0x7fffffffe000))

    int main()
    {
    int fds[2], err;
    void * ptr = mmap(LAST_PAGE, PAGE_SIZE, PROT_READ | PROT_WRITE,
    MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED, -1, 0);
    assert(ptr == LAST_PAGE);
    err = socketpair(AF_LOCAL, SOCK_STREAM, 0, fds);
    assert(err == 0);
    err = send(fds[0], ptr, PAGE_SIZE, 0);
    perror("send");
    assert(err == PAGE_SIZE);
    err = recv(fds[1], ptr, PAGE_SIZE, MSG_WAITALL);
    perror("recv");
    assert(err == PAGE_SIZE);
    return 0;
    }
    ---

    Other place checking the addr limit is access_ok function,
    which is working properly. There's just misleading comment
    for the __range_not_ok macro.


    Signed-off-by: Jiri Olsa <jolsa@redhat.com>
    ---
    arch/x86/include/asm/uaccess.h | 2 +-
    arch/x86/lib/copy_user_64.S | 4 ++--
    2 files changed, 3 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
    index abd3e0e..99f0ad7 100644
    --- a/arch/x86/include/asm/uaccess.h
    +++ b/arch/x86/include/asm/uaccess.h
    @@ -42,7 +42,7 @@
    * Returns 0 if the range is valid, nonzero otherwise.
    *
    * This is equivalent to the following test:
    - * (u33)addr + (u33)size >= (u33)current->addr_limit.seg (u65 for x86_64)
    + * (u33)addr + (u33)size > (u33)current->addr_limit.seg (u65 for x86_64)
    *
    * This needs 33-bit (65-bit for x86_64) arithmetic. We have a carry...
    */
    diff --git a/arch/x86/lib/copy_user_64.S b/arch/x86/lib/copy_user_64.S
    index 99e4826..a73397f 100644
    --- a/arch/x86/lib/copy_user_64.S
    +++ b/arch/x86/lib/copy_user_64.S
    @@ -72,7 +72,7 @@ ENTRY(_copy_to_user)
    addq %rdx,%rcx
    jc bad_to_user
    cmpq TI_addr_limit(%rax),%rcx
    - jae bad_to_user
    + ja bad_to_user
    ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
    CFI_ENDPROC
    ENDPROC(_copy_to_user)
    @@ -85,7 +85,7 @@ ENTRY(_copy_from_user)
    addq %rdx,%rcx
    jc bad_from_user
    cmpq TI_addr_limit(%rax),%rcx
    - jae bad_from_user
    + ja bad_from_user
    ALTERNATIVE_JUMP X86_FEATURE_REP_GOOD,copy_user_generic_unrolled,copy_user_generic_string
    CFI_ENDPROC
    ENDPROC(_copy_from_user)
    --
    1.7.1


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