On Thu, Aug 17, 2017 at 3:35 AM, Will Deacon <will.deacon@xxxxxxx> wrote: > On Tue, Aug 15, 2017 at 11:35:41PM +0100, Mark Rutland wrote: >> On Wed, Aug 16, 2017 at 09:14:41AM -0700, Andy Lutomirski wrote: >> > On Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote: >> > > On Wed, 16 Aug, at 12:03:22PM, Mark Rutland wrote: >> > >> >> > >> I'd expect we'd abort at a higher level, not taking any sample. i.e. >> > >> we'd have the core overflow handler check in_funny_mm(), and if so, skip >> > >> the sample, as with the skid case. >> > > >> > > FYI, this is my preferred solution for x86 too. >> > >> > One option for the "funny mm" flag would be literally the condition >> > current->mm != current->active_mm. I *think* this gets all the cases >> > right as long as efi_switch_mm is careful with its ordering and that >> > the arch switch_mm() code can handle the resulting ordering. (x86's >> > can now, I think, or at least will be able to in 4.14 -- not sure >> > about other arches). >> >> For arm64 we'd have to rework things a bit to get the ordering right >> (especially when we flip to/from the idmap), but otherwise this sounds sane to >> me. >> >> > That being said, there's a totally different solution: run EFI >> > callbacks in a kernel thread. This has other benefits: we could run >> > those callbacks in user mode some day, and doing *that* in a user >> > thread seems like a mistake. >> >> I think that wouldn't work for CPU-bound perf events (which are not >> ctx-switched with the task). >> >> It might be desireable to do that anyway, though. > > I'm still concerned that we're treating perf specially here -- are we > absolutely sure that nobody else is going to attempt user accesses off the > back of an interrupt? Reasonably sure? If nothing else, an interrupt taken while mmap_sem() is held for write that tries to access user memory is asking for serious trouble. There are still a few callers of pagefault_disable() and copy...inatomic(), though. > If not, then I'd much prefer a solution that catches > anybody doing that with the EFI page table installed, rather than trying > to play whack-a-mole like this. Using a kernel thread solves the problem for real. Anything that blindly accesses user memory in kernel thread context is terminally broken no matter what. > > Will -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
