Re: [RFC PATCH 07/10] uaccess: Fail user nofault helpers quietly when current->mm is NULL

[sashiko-bot@kernel.org]

> The user nofault helpers (copy_{from,to}_user_nofault(),
> strncpy_from_user_nofault() and strnlen_user_nofault()) are meant to be
> callable from any context, e.g. from BPF programs, kprobes or perf, which
> may run on behalf of a kernel thread or of a task that has already gone
> through exit_mm(). In such a context, there is no user memory to access,
> and the right answer is simply to fail.
> 
> Since the previous commit, the uaccess helpers they use warn when
> current->mm is NULL, e.g. bpf_probe_read_user_str() from a BPF program
> attached to a function called by a kernel thread would now trip the
> WARN_ON_ONCE() in strncpy_from_user().
> 
> Check for a NULL current->mm up front in these helpers, and fail without
> warning and without attempting the access.
> 
> On x86, copy_from_user_nofault() was already refusing such accesses, as
> nmi_uaccess_okay() returns false when current->mm is not the loaded mm.
> 
> For the same reason, make copy_to_user_iter_nofault() use
> should_fail_usercopy() directly, leaving the NULL mm check to
> copy_to_user_nofault().
> 
> Signed-off-by: James Houghton <jthoughton@google.com>

Sashiko has reviewed this patch and found no issues.

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Sashiko AI review · https://sashiko.dev/#/patchset/20261026215226.3713941-1-jthoughton@google.com?part=7