Re: [kernel-hardening] [RFC] x86, mm: start mmap allocation for libs from low addresses

[Solar Designer <solar@openwall.com>]

On Wed, Sep 07, 2011 at 01:55:08PM +0400, Vasiliy Kulikov wrote:
> OK, fully agree.  But why 100 KB?  Probably 0x10000 (64 KB)?  It looks
> nicer and not so magic.

Well, on Owl we have mmap_min_addr at 96 KB, which is sufficient e.g. in
case we have a struct field offset not larger than 32 KB and the field
itself is an array indexed by a 16-bit value.  Or if the field offset is
not larger than 64 KB and the index is a signed 16-bit value.

100 KB is a very cheap enhancement of the above, also allowing for two
levels of indirection (up to one 16-bit signed and one 16-bit unsigned)
relative to a fixed offset that fits in 4 KB.

Maybe we should move from 96 KB to 100 KB for Owl's mmap_min_addr
default.  Or maybe we should use 132 KB (4+64+64).

Oh, this assumes arrays of char, or our 16-bit variable being byte
offset rather than index.  132 KB would also support arrays of 16-bit
words, and even 16-bit signed indexes into arrays of 32-bit words.

OK, maybe I am imagining these possibilities, but to me these values
feel a little bit more reasonable than a mere 64 KB, which might be
just insufficient e.g. if we have a 16-bit unsigned byte offset variable
and the array itself is a struct field.  Even 68 KB would be a lot more
likely to help then.

Alexander