linux-kernel - Re: [PATCH v3] riscv: fix kprobe __user string arg print fault issue

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Date:   Fri, 02 Jun 2023 11:41:13 +0200
From:   Björn Töpel <bjorn@...nel.org>
To:     Ruan Jinjie <ruanjinjie@...wei.com>, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        palmer@...osinc.com
Subject: Re: [PATCH v3] riscv: fix kprobe __user string arg print fault issue

Ruan Jinjie <ruanjinjie@...wei.com> writes:

> Ping.

Maybe it's not clear *why* we need
ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE on RISC-V?

ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE is used in the kernel to
determine what "memory access" function to use, e.g.

  | #ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
  |         if ((unsigned long)str_val < TASK_SIZE)
  |             ret = strncpy_from_user_nofault(str_field, str_val, STR_VAR_LEN_MAX);
  |         else
  | #endif
  |             ret = strncpy_from_kernel_nofault(str_field, str_val, STR_VAR_LEN_MAX);

RISC-V makes use of the SUM bit [1], which requires the kernel flips a
bit explicitly to touch user memory, so it's important to use the
correct access function.

What this means, is that if
CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE is *not* defined for RV
kernels, the copy_from_kernel will fault (as Jinjie points out in the
commit message).

The fixes tag should be when Daniel Borkmann introduced the config -- or
that makes sense from a backport perspective.

Maybe Palmer can add a lore link to this post, and the following
fixes-tag

  Fixes: 0ebeea8ca8a4 ("bpf: Restrict bpf_probe_read{, str}() only to archs where they work")

when applying?

Björn

[1] https://github.com/riscv/riscv-isa-manual/blob/main/src/supervisor.adoc?plain=1#L118