On Mon, Feb 14, 2022 at 08:17:07PM +0000, Al Viro wrote: > On Mon, Feb 14, 2022 at 12:01:05PM -0800, Linus Torvalds wrote: > > On Mon, Feb 14, 2022 at 11:46 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > > > > > As Al pointed out, they turned out to be necessary on sparc64, but the only > > > definitions are on sparc64 and x86, so it's possible that they serve a similar > > > purpose here, in which case changing the limit from TASK_SIZE to > > > TASK_SIZE_MAX is probably wrong as well. > > > > x86-64 has always(*) used TASK_SIZE_MAX for access_ok(), and the > > get_user() assembler implementation does the same. > > > > I think any __range_not_ok() users that use TASK_SIZE are entirely > > historical, and should be just fixed. > > IIRC, that was mostly userland stack trace collection in perf. > I'll try to dig in archives and see what shows up - it's been > a while ago... After some digging: access_ok() needs only to make sure that MMU won't go anywhere near the kernel page tables; address limit for 32bit threads is none of its concern, so TASK_SIZE_MAX is right for it. valid_user_frame() in arch/x86/events/core.c: used while walking the userland call chain. The reason it's not access_ok() is only that perf_callchain_user() might've been called from interrupt that came while we'd been under KERNEL_DS. That had been back in 2015 and it had been obsoleted since 2017, commit 88b0193d9418 (perf/callchain: Force USER_DS when invoking perf_callchain_user()). We had been guaranteed USER_DS ever since. IOW, it could've reverted to use of access_ok() at any point after that. TASK_SIZE vs TASK_SIZE_MAX is pretty much an accident there - might've been TASK_SIZE_MAX from the very beginning. copy_stack_frame() in arch/x86/kernel/stacktrace.c: similar story, except the commit that made sure callers will have USER_DS - cac9b9a4b083 (stacktrace: Force USER_DS for stack_trace_save_user()) in this case. Also could've been using access_ok() just fine. Amusingly, access_ok() used to be there, until it had been replaced with explicit check on Jul 22 2019 - 4 days after that had been made useless by fix in the caller... copy_from_user_nmi(). That one is a bit more interesting. We have a call chain from perf_output_sample_ustack() (covered by force_uaccess_begin() these days, not that it mattered for x86 now), there's something odd in dumpstack.c:copy_code() (with explicit check for TASK_SIZE_MAX in the caller) and there's a couple of callers in Intel PMU code. AFAICS, there's no reason whatsoever to use TASK_SIZE in that one - the point is to prevent copyin from the kernel memory, and in that respect TASK_SIZE_MAX isn't any worse. The check in copy_code() probably should go. So all of those guys should be simply switched to access_ok(). Might be worth making that a preliminary patch - it's independent from everything else and there's no point folding it into any of the patches in the series.
