On Wed, Apr 21, 2010 at 5:44 AM, Konstantin Belousov <[email protected]>wrote:
>
> I think that access validation currently done by msync(2) is not enough
> for rare case of unaligned access at the end of the page with next page
> unmapped.
>
> Also, mincore(2) looks like as a better way to test the presence of a
> mapping at some address. I use the patch below.
>
>
The reason why I picked msync instead of mincore had to do with minor
faults. If the address can be accessed with a minor fault, mincore returns
false and we end up doing validation that was not strictly necessary.
The linux kernel implementation of mincore() involves:
/* ..and we need to be passed a valid user-space range */
if (!access_ok(VERIFY_READ, (void __user *) start, len))
return -ENOMEM;
In the ideal world, what validate_mem() wants is the
access_ok(VERIFY_READ,..) syscall.
Perhaps we can ignore the output value (mvec) and just test for errno ==
ENOMEM. Wait, that's what your patch is doing (mostly) :)
Another corner case we need to test: check if the technique works for
PROT_NONE regions (msync says its ok to access when in fact it isn't safe).
-Arun
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