Re: Expected rdpmc behavior during context swtich and a RISC-V conundrum

[Mark Rutland <mark.rutland@arm.com>]

On Thu, Jan 05, 2023 at 11:59:24AM -0800, Atish Patra wrote:
> Hi All,
> There was a recent uabi update[1] for RISC-V that allows the users to
> read cycle and instruction count without any checks.
> We tried to restrict that behavior to address security concerns
> earlier but it resulted in breakage for some user space
> applications[2].
> Thus, previous behavior was restored where a user on RISC-V platforms
> can directly read cycle or instruction count[3].
> 
> Comparison with other ISAs w.r.t user space access of counters:
> ARM64
>   -- Enabled/Disabled via (/proc/sys/kernel/perf_user_access)
>   -- Only for task bound events configured via perf.
> 
> X86
>  --- rdpmc instruction
>  --- Enable/Disable via “/sys/devices/cpu/rdpmc”
> -- Before v4.0
>  -- any process (even without active perf event) rdpmc
> After v4.0
> -- Default behavior changed to support only active events in a
> process’s context.
> -- Configured through perf similar to ARM64
> -- Continue to maintain backward compatibility for unrestricted access
> by writing 2 to “/sys/devices/cpu/rdpmc”
> 
> IMO, RISC-V should only enable user space access through perf similar
> to ARM64 and x86 (post v4.0).
> However, we do have to support the legacy behavior to avoid
> application breakage.
> As per my understanding a direct user space access can lead to the
> following problems:
> 
> 1) There is no context switch support, so counts from other contexts are exposed
> 2) If a perf user is allocated one of these counters, the counter
> value will be written
> 
> Looking at the x86 code as it continues to allow the above behavior,
> rdpmc_always_available_key is enabled in the above case. However,
> during the context switch (cr4_update_pce_mm)
> only dirty counters are cleared. It only prevents leakage from perf
> task to rdpmc task.
> 
> How does the context switch of counters work for users who enable
> unrestricted access by writing 2 to “/sys/devices/cpu/rdpmc” ?
> Otherwise, rdpmc users likely get noise from other applications. Is
> that expected ?

Regardless of leakage, they're also going to get random jumps in the visible
values of the cycle count and instruction count as the task is context-switched
(and/or if those values get reset across idle, as can happen on arm64), so
those aren't going to be useful unless a number of other constraints apply.

AFAICT the affected package was actually a library of intrinsics; does this
affect a real application, or was this just in tests? If it's the latter there
might still be scope to properly lock this down...

Thanks,
Mark.

> This can be a security concern also where a rogue rdpmc user
> application can monitor other critical applications to initiate side
> channel attack.
> 
> Am I missing something? Please correct my understanding of the x86
> implementation if it is wrong.
> 
> [1] https://lore.kernel.org/lkml/20221201135110.3855965-1-conor.dooley@microchip.com/
> [2] https://groups.google.com/a/groups.riscv.org/g/sw-dev/c/REWcwYnzsKE?pli=1
> [3] https://lore.kernel.org/all/YxIzgYP3MujXdqwj@aurel32.net/T/
> 
> -- 
> Regards,
> Atish