Messages in this thread | | | Date | Wed, 6 Mar 2019 00:18:11 +0900 | From | Masami Hiramatsu <> | Subject | Re: [uaccess] 780464aed0: WARNING:at_arch/x86/include/asm/uaccess.h:#strnlen_user/0x |
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On Tue, 5 Mar 2019 15:53:06 +0100 Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Mar 05, 2019 at 10:58:01PM +0900, Masami Hiramatsu wrote: > > > Could you tell me why WARN_ON_ONCE(!in_task()) is needed in access_ok()? > > That came from here: > > lkml.kernel.org/r/20190225145240.GB32534@hirez.programming.kicks-ass.net > > Because in-irq usage is dodgy, since we don't actually know what mm or > ds it loaded.
Yes, I would like to allow it only if setting pagefault-disable correctly. (and setting ds too, it is good to me)
> > > > I dislike that whole KERNEL_DS thing, but obviously that's not something > > > that's going away. > > > > > > Would something like: > > > > > > WARN_ON_ONCE(!(in_task || segment_eq(get_fs(), USER_DS))) > > > > > > Work? Then we allow KERNEL_DS in task context, but for interrupt and > > > others require USER_DS. > > > > But what would this mean? I can't understand why we limit using > > access_ok() so strictly and narrow the cases. > > Because it's been a source of bugs. Any sanity checking we can put in > seems like a good thing at this point.
Hmm, I see yours is strict, fit with current code, but complicated rule.
- strncpy_from_user() can access user memory with set_fs(USER_DS) in task context
- strncpy_from_user() can access kernel memory with set_fs(KERNEL_DS) in task context
- strncpy_from_user() can access user memory in IRQ context if pagefault is disabled and with set_fs(USER_DS). (but pagefault-disabled is not verified)
- strncpy_from_user() never allowed to access kernel memory in IRQ context, even if pagefault is disabled and with set_fs(KERNEL_DS).
And mine is simple.
- strncpy_from_user() can access user memory with set_fs(USER_DS) in task context
- strncpy_from_user() can access kernel memory with set_fs(KERNEL_DS) in task context
- strncpy_from_user() can access user/kernel memory (depends on DS) in IRQ context if pagefault is disabled. (both verified)
Thank you,
-- Masami Hiramatsu <mhiramat@kernel.org>
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