Re: Access unprivileged regions from OS

Bill Huffman

On 10/30/20 2:32 PM, Andrew Waterman wrote:

On Fri, Oct 30, 2020 at 8:19 AM Xinhaoqu (Freddie) <xinhaoqu@...> wrote:

Hi Andrew,


I’m not sure the sstatus.SUM bit is providing the equivalent of LDTR/STTR. The pair of load/store instructions lower their privilege level so that if they end up access privileged locations, they will fault. On the other hand, when status.SUM==1, even if the page is marked as “User”, supervisor code can still access it.


From section in the Privileged ISA spec:


The SUM (permit Supervisor User Memory access) bit modifies the privilege with which S-mode

loads and stores access virtual memory. When SUM=0, S-mode memory accesses to pages that are

accessible by U-mode (U=1 in Figure 4.17) will fault. When SUM=1, these accesses are permitted.

SUM has no effect when page-based virtual memory is not in effect. Note that, while SUM is

ordinarily ignored when not executing in S-mode, it is in effect when MPRV=1 and MPP=S. SUM

is hardwired to 0 if S-mode is not supported.


There is nothing mentioning what would happen if load/store instructions in S-mode attempt locations that require privilege. That means to me they are permitted when sstatus.SUM==1. This behaviour is fine in itself, but doesn’t match what the LDTR/STTR instructions do. I think sstatus.SUM’s equivalent is PSTATE.PAN, not LDTR/STTR. In other words, LDTR/STTR has no equivalent in RISC-V, AFAIK.

Right.  SUM can be used to solve the same problem as LDTR/STTR, but it is not equivalent.  To avoid the concern you describe, the Linux kernel first performs a bounds check to guarantee the address is in the user process' VA range.  Then, it engages SUM and performs the unprivileged access.

That's, of course, a very Linux centric answer.  With more general address maps - or a "bare MMU" - the problem is harder to solve.  Do you have an expectation for that case?







From: tech-privileged@... [mailto:tech-privileged@...] On Behalf Of Andrew Waterman
Sent: Friday, October 30, 2020 9:50 AM
To: Andrea Mondelli <andrea.mondelli@...>
Cc: tech-privileged@...
Subject: Re: [RISC-V] [tech-privileged] Access unprivileged regions from OS




On Fri, Oct 30, 2020 at 2:45 AM Andrea Mondelli via <> wrote:

Hi all,

quoting the arm manual, "sometimes the OS does need to access unprivileged regions, for example, to write to a buffer owned by an application. To support this, the instruction set provides the LDTR and STTR instructions."
According to the Volume II: RISC-V Privileged Architectures Chapter 7, In RISCV we don't have any similar privileged instruction to do it.

There is an alternative way to have the same behavior? I was thinking other examples like checking user parameters when syscall are called.


Yeah.  Set the sstatus.SUM bit, then use regular load and store instructions to access user memory, then clear sstatus.SUM.


Any hints?

thanks in advance

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