Re: [PATCH v11 9/9] x86: Disallow vsyscall emulation when CET is enabled

* Andy Lutomirski:

On Mon, Aug 24, 2020 at 5:30 PM Yu-cheng Yu [off-list ref] wrote:

quoted

From: "H.J. Lu" <redacted>

Emulation of the legacy vsyscall page is required by some programs built
before 2013.  Newer programs after 2013 don't use it.  Disallow vsyscall
emulation when Control-flow Enforcement (CET) is enabled to enhance
security.

NAK.

By all means disable execute emulation if CET-IBT is enabled at the
time emulation is attempted, and maybe even disable the vsyscall page
entirely if you can magically tell that CET-IBT will be enabled when a
process starts, but you don't get to just disable it outright on a
CET-enabled kernel.

Yeah, we definitely would have to revert/avoid this downstream.  People
definitely want to run glibc-2.12-era workloads on current kernels.
Thanks for catching it.

Florian

On 8/25/2020 2:14 AM, Florian Weimer wrote:

* Andy Lutomirski:

quoted

On Mon, Aug 24, 2020 at 5:30 PM Yu-cheng Yu [off-list ref] wrote:

quoted

From: "H.J. Lu" <redacted>

Emulation of the legacy vsyscall page is required by some programs built
before 2013.  Newer programs after 2013 don't use it.  Disallow vsyscall
emulation when Control-flow Enforcement (CET) is enabled to enhance
security.

NAK.

By all means disable execute emulation if CET-IBT is enabled at the
time emulation is attempted, and maybe even disable the vsyscall page
entirely if you can magically tell that CET-IBT will be enabled when a
process starts, but you don't get to just disable it outright on a
CET-enabled kernel.

Yeah, we definitely would have to revert/avoid this downstream.  People
definitely want to run glibc-2.12-era workloads on current kernels.
Thanks for catching it.

That makes sense.  I will update the patch.

Thanks,
Yu-cheng