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    SubjectRe: [RFC] prevent "dd if=/dev/mem" crash
    Date
    On Friday 17 October 2003 5:55 pm, Andrew Morton wrote:
    > Still, the code you have is quite reasonable. But please structure it
    > thusly:
    > ...

    Here's a patch structured that way.

    > As for return values: if the requested read or write starts at a
    > not-present address it should probably return -EFAULT. This is what ia32
    > will do. Arguably this is indistinguishable from a bad address on the
    > userspace side and we should return -EINVAL but whatever.

    I made it return -EFAULT. I worry a little bit because ia32
    returned 0 (short read) when (addr >= high_memory) before,
    but I don't have a strong opinion one way or the other.

    ===== drivers/char/mem.c 1.44 vs edited =====
    --- 1.44/drivers/char/mem.c Sun Sep 21 15:50:34 2003
    +++ edited/drivers/char/mem.c Mon Oct 20 10:43:08 2003
    @@ -79,6 +79,22 @@
    #endif
    }

    +#ifndef ARCH_HAS_VALID_PHYS_ADDR_RANGE
    +static inline int valid_phys_addr_range(unsigned long addr, size_t *count)
    +{
    + unsigned long end_mem;
    +
    + end_mem = __pa(high_memory);
    + if (addr >= end_mem)
    + return 0;
    +
    + if (*count > end_mem - addr)
    + *count = end_mem - addr;
    +
    + return 1;
    +}
    +#endif
    +
    static ssize_t do_write_mem(struct file * file, void *p, unsigned long realp,
    const char * buf, size_t count, loff_t *ppos)
    {
    @@ -113,14 +129,10 @@
    size_t count, loff_t *ppos)
    {
    unsigned long p = *ppos;
    - unsigned long end_mem;
    ssize_t read;

    - end_mem = __pa(high_memory);
    - if (p >= end_mem)
    - return 0;
    - if (count > end_mem - p)
    - count = end_mem - p;
    + if (!valid_phys_addr_range(p, &count))
    + return -EFAULT;
    read = 0;
    #if defined(__sparc__) || (defined(__mc68000__) && defined(CONFIG_MMU))
    /* we don't have page 0 mapped on sparc and m68k.. */
    @@ -149,13 +161,9 @@
    size_t count, loff_t *ppos)
    {
    unsigned long p = *ppos;
    - unsigned long end_mem;

    - end_mem = __pa(high_memory);
    - if (p >= end_mem)
    - return 0;
    - if (count > end_mem - p)
    - count = end_mem - p;
    + if (!valid_phys_addr_range(p, &count))
    + return -EFAULT;
    return do_write_mem(file, __va(p), p, buf, count, ppos);
    }

    -
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