From: Siddhesh Poyarekar <siddhesh@gotplt.org>
To: Topi Miettinen <toiwoton@gmail.com>, libc-alpha@sourceware.org
Subject: Re: Randomize offset between program segments?
Date: Wed, 21 Oct 2020 07:51:32 +0530 [thread overview]
Message-ID: <e56acd8c-5c18-5e0c-25d5-56e9e15c3e72@gotplt.org> (raw)
In-Reply-To: <f7ce6a45-358e-f024-def3-ef3c223a241e@gmail.com>
On 10/19/20 9:01 PM, Topi Miettinen via Libc-alpha wrote:
> I'd like to improve address space randomization (ASLR) by randomizing
> the offset between .text, .data and .bss segments (or more generalized,
> any program segments). With large code generation model (-mcmodel=large)
> on AMD64, the offset could be very large, but even with the default
> model, the segments could be randomized within range of RIP-relative
> accesses (+/-2GB). Currently the dynamic loader can't randomize the
> segments (nothing also tells it if this would be OK) so it maps them
> next to each other, which is predictable and boring.
What is the advantage of randomizing offsets between these segments?
> For this to happen, I think the compiler would have to emit relocations
> for all cross-segment accesses and probably flagging the shared object
> somehow. Then, when detecting the flag, the dynamic loader could load
> the segments at random offsets within 2GB, or if the large model was
> used in compilation (another flag), anywhere in the available virtual
> address space (let OS map the segment anywhere by using mmap(NULL,...)).
The challenge is not just pc-relative range, but also technical
feasibility of using pc-relative loads in the first place because the
change would either involve an additional indirection during load (like
GOT) which could have a significant performance overhead or a text
relocation, which would be like inviting wolves to drive out raccoons.
Of course, first we need to determine if there are raccoons.
Siddhesh
next prev parent reply other threads:[~2020-10-21 2:21 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-19 15:31 Topi Miettinen
2020-10-21 2:21 ` Siddhesh Poyarekar [this message]
2020-10-21 7:40 ` Topi Miettinen
2020-10-21 9:14 ` Siddhesh Poyarekar
2020-10-21 9:34 ` Topi Miettinen
2020-10-21 9:54 ` Siddhesh Poyarekar
2020-10-21 10:44 ` Topi Miettinen
2020-10-29 8:04 ` Siddhesh Poyarekar
2020-10-30 15:37 ` Topi Miettinen
2020-10-21 5:33 ` Florian Weimer
2020-10-21 8:02 ` Topi Miettinen
2020-10-21 8:06 ` Florian Weimer
2020-10-21 9:28 ` Topi Miettinen
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