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    Subject[RFC] prevent "dd if=/dev/mem" crash
    Date
    This is the generic part of a change to prevent "dd if=/dev/mem"
    from causing a machine check on ia64.

    read_mem() and write_mem() already check the requested address
    against "high_memory", but that is only a complete check if
    everything from 0 to high_memory is valid, readable/writable
    memory. Obviously that's not the case for architectures with
    discontiguous memory, like ia64.

    Old behavior:

    # dd if=/dev/mem of=/dev/null
    <unrecoverable machine check>

    New behavior (this system has a hole from 0-16MB, then memory
    from 16MB-1GB):

    # dd if=/dev/mem of=/dev/null
    0+0 records in
    0+0 records out
    0 bytes transferred in 0.000282 seconds (0 bytes/sec)

    # dd if=/dev/mem of=/dev/null bs=1M skip=16
    1004+10 records in
    1004+10 records out
    1056964608 bytes transferred in 1.629262 seconds (648738280 bytes/sec)

    I expect there are probably different opinions about the idea
    that "dd if=/dev/mem" exits without doing anything. Sparc and
    68K have nearby code that bit-buckets writes and returns zeroes
    for reads of page zero. We could do that, too, but it seems like
    kind of a hack, and holes on ia64 can be BIG (on the order of
    256GB for one box).

    So flame away :-)

    The patch below is mangled so it won't apply easily. If this
    seems a reasonable approach, I'll submit the ia64 piece first,
    then repost this.

    Bjorn

    ===== 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 Fri Oct 17 15:37:47 2003
    @@ -79,6 +79,24 @@
    #endif
    }

    +static inline int valid_mem_range(unsigned long addr, size_t *count)
    +{
    +#if defined(CONFIG_IA64)
    + return efi_valid_mem_range(addr, count);
    +#else
    + 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 +131,10 @@
    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)
    + if (!valid_mem_range(p, &count))
    return 0;
    - if (count > end_mem - p)
    - count = end_mem - p;
    read = 0;
    #if defined(__sparc__) || (defined(__mc68000__) && defined(CONFIG_MMU))
    /* we don't have page 0 mapped on sparc and m68k.. */
    @@ -149,13 +163,9 @@
    size_t count, loff_t *ppos)
    {
    unsigned long p = *ppos;
    - unsigned long end_mem;

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

    ===== include/linux/efi.h 1.3 vs edited =====
    --- 1.3/ include/linux/efi.h Thu Aug 7 14:01:48 2003
    +++ edited/ include/linux/efi.h Thu Oct 16 16:54:52 2003
    @@ -266,6 +266,7 @@
    extern u64 efi_get_iobase (void);
    extern u32 efi_mem_type (unsigned long phys_addr);
    extern u64 efi_mem_attributes (unsigned long phys_addr);
    +extern int efi_valid_mem_range (unsigned long phys_addr, unsigned long *count);

    /*
    * Variable Attributes
    -
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