[PATCH] permission hook in filemap_nopage

aedwardat_private

Hi,

At the moment there is no permission check in filemap_nopage (reading in a
page from an
mmaped file). This makes the permission check in sys_read / sys_write a
little pointless
as the user can always avoid this check by mmaping the file.

I've attaced a patch that puts in a hook at the top of filemap_nopage for
consideration. I've
also included some notes about the various choices I made below.

Cheers,

Antony(See attached file: filemap-2.4.16.patch)

Notes:

(1) The access bits passed to the permission hook are from the
vm_area_struct rather
than the file (though they are converted to the correct format). Thus if
the user opens the
file O_RDWR by maps it PROT_READ -- the permission check will be for
O_RDONLY. This
seemed the best way to go -- however the other way is just as easy if
someone comes up with
a good reason.

(2) An protection error results in a SIGBUS. I was hoping to do a SIGSEGV,
however, the
number of extra modifications required to achieve this aren't really
justified.

(3) If the area is mapped PROT_READ | PROT_WRITE -- both are checked when
the file
is first mapped in. I.e., I don't only check PROT_READ on filemap_nopage
and then check
PROT_WRITE if they actually try and write to the page. This requires an
extra check and
I can't see the benefit.

(4) If the patch is accepted, I think there should be a comment in
security.h informing module
writers that file_ops->permission could get called from a pagefault handler
-- so be careful.