Messages in this thread | | | Subject | Re: arch_set_user_pkey_access only works on the current task_struct | From | Dave Hansen <> | Date | Tue, 8 Jun 2021 07:55:29 -0700 |
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On 6/7/21 8:16 PM, liangjs wrote: > On Mon, 2021-06-07 at 10:52 -0700, Dave Hansen wrote: >> On 6/5/21 6:10 AM, Jiashuo Liang wrote: >>> I am learning the kernel implementation of the x86 PKU feature. I find the >>> arch_set_user_pkey_access function in arch/x86/kernel/fpu/xstate.c does not >>> use its first parameter. So it is perhaps a bug? >> I wouldn't really call it a bug. But, yes, it is something we should >> clean up. > Should we remove the tsk parameter, or allow it to change the PKRU of tsk?
Probably just remove the parameter.
By the way, there's a big PKRU rework in progress. It might be best to wait until the dust settles to poke at this.
> By the way, we are calling write_pkru, which changes both the CPU's PKRU > and the xsave one. Why is this necessary?
PKRU affects kernel accesses to user memory. That means that you can't run the *kernel* with an out-of-date PKRU, thus the write_pkru().
Returning to userspace blindly restores the *WHOLE* XSAVE buffer to the regsisters. If you don't update the XSAVE buffer, the write_pkru() will be overwritten before returning to userspace.
> If I want to change PKRU of a task_struct other than current, do I still > need to call __write_pkru?
No. You can't do that. Seriously.
The protection keys architecture really doesn't support off-thread manipulation of PKRU. Imagine you want to mask a bit out of PKRU, you do the following to make key 2 memory accessible and writable:
reg = read_pkru(); reg &= 0x30; write_pkru(reg);
Now, imagine that you tried to interrupt this poor task in the middle of that operation. Let's say you try to *set* the bits for key 4, effectively:
pkru |= 0x300;
Now you try to do that key-4 business with an IPI.
reg = read_pkru(); // PKRU=0x30 reg &= 0x30; -> IPI ipireg = read_pkru(); // PKRU=0x0 ipireg |= 0x300; write_pkru(ipireg); // PKRU=0x300 write_pkru(reg); // PKRU=0x0
You *LOST* the update from the IPI.
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