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[209.85.167.48]) by smtp.gmail.com with ESMTPSA id a70sm2655096ljf.57.2019.07.06.17.08.37 for (version=TLS1_3 cipher=AEAD-AES128-GCM-SHA256 bits=128/128); Sat, 06 Jul 2019 17:08:37 -0700 (PDT) Received: by mail-lf1-f48.google.com with SMTP id p197so8522493lfa.2 for ; Sat, 06 Jul 2019 17:08:37 -0700 (PDT) X-Received: by 2002:ac2:59c9:: with SMTP id x9mr5027021lfn.52.1562458116980; Sat, 06 Jul 2019 17:08:36 -0700 (PDT) MIME-Version: 1.0 References: <20190704195555.580363209@infradead.org> <20190704200050.534802824@infradead.org> <20190705134916.GU3402@hirez.programming.kicks-ass.net> <20190706182728.435a89ed@gandalf.local.home> In-Reply-To: From: Linus Torvalds Date: Sat, 6 Jul 2019 17:08:20 -0700 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH v2 5/7] x86/mm, tracing: Fix CR2 corruption To: Steven Rostedt Cc: Peter Zijlstra , Thomas Gleixner , Borislav Petkov , Ingo Molnar , Andrew Lutomirski , Peter Anvin , Dave Hansen , Juergen Gross , Linux List Kernel Mailing , He Zhe , Joel Fernandes , devel@etsukata.com Content-Type: multipart/mixed; boundary="0000000000000a3535058d0c21d7" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --0000000000000a3535058d0c21d7 Content-Type: text/plain; charset="UTF-8" On Sat, Jul 6, 2019 at 3:41 PM Linus Torvalds wrote: > > On Sat, Jul 6, 2019 at 3:27 PM Steven Rostedt wrote: > > > > We also have to deal with reading vmalloc'd data as that can fault too. > > Ahh, that may be a better reason for PeterZ's patches and reading cr2 > very early from asm code than the stack trace case. Hmm. Another alternative might be to simply just make our vmalloc page fault handling more robust. Right now, if we take a vmalloc page fault in an inconvenient spot, it is fatal because it corrupts the cr2 in the outer context. However, it doesn't *need* to be fatal. Who cares if the outer context cr2 gets corrupted? We probably *shouldn't* care - it's an odd and unusual case, and the outer context could just handle the wrong vmalloc-address cr2 fine (it's going to be a no-op, since the inner page fault will have handled it already), return, and then re-fault. The only reason it's fatal right now is that we care much too deeply about (a) the error code (b) the pt_regs state when we handle vmalloc faults. So one option is that we simply handle the vmalloc faults _without_ caring about the error code and the pt_regs state, because even if the error code or the pt_regs implies that the fault comes from user space, the cr2 value might be due to a vmalloc fault from the inner kernel page fault that corrupted cr2. Right now vmalloc faults are already special and need to be handled without holding any locks etc. We'd just make them even more special, and say that we might have a vmalloc area fault from any context. IOW, somethinig like the attached patch would make nesting vmalloc faults harmless. Sure, we'll do the "vmalloc fault" twice, and return and re-do the original page fault, but this is a very unusual case, so from a performance angle we don't really care. But I guess the "read cr2 early" is fine too.. 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