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Lu" Date: Fri, 10 Mar 2023 12:00:45 -0800 Message-ID: Subject: Re: [PATCH v7 28/41] x86: Introduce userspace API for shadow stack To: "Edgecombe, Rick P" Cc: "bp@alien8.de" , "joao@overdrivepizza.com" , "david@redhat.com" , "bsingharora@gmail.com" , "hpa@zytor.com" , "Syromiatnikov, Eugene" , "peterz@infradead.org" , "rdunlap@infradead.org" , "keescook@chromium.org" , "Eranian, Stephane" , "kirill.shutemov@linux.intel.com" , "dave.hansen@linux.intel.com" , "linux-mm@kvack.org" , "fweimer@redhat.com" , "nadav.amit@gmail.com" , "jannh@google.com" , "dethoma@microsoft.com" , "kcc@google.com" , "linux-arch@vger.kernel.org" , "pavel@ucw.cz" , "oleg@redhat.com" , "akpm@linux-foundation.org" , "Lutomirski, Andy" , "jamorris@linux.microsoft.com" , "arnd@arndb.de" , "tglx@linutronix.de" , "Schimpe, Christina" , "mike.kravetz@oracle.com" , "debug@rivosinc.com" , "Yang, Weijiang" , "x86@kernel.org" , "andrew.cooper3@citrix.com" , "john.allen@amd.com" , "linux-doc@vger.kernel.org" , "rppt@kernel.org" , "mingo@redhat.com" , "corbet@lwn.net" , "linux-kernel@vger.kernel.org" , "linux-api@vger.kernel.org" , "gorcunov@gmail.com" Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: a8sw8h78666jc7hmqab39wp137xfeqjc X-Rspamd-Queue-Id: F32348000C X-HE-Tag: 1678478481-236849 X-HE-Meta: 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 jkcn3H0W Z0vNtazbAYNLtz/QItCA45tEkqh2Ja1rDloH959fAaiXPYAjLo9aIwC+oinXsk0lDWI+Is2A3/HgUDnFiMlc0IdBkRFNOO/shxMMA1DU+dhDKcBlJBFTnyTVTrtJJxmTxxrmvp5wGDRxqa3T45RcEgnjYLDFUhkayeWIWIWcvV98hSqaY/3z1k2cEIC9uoWVgqnaU2Un2fAiM/ciXwZE5s4qhh8wg641hnDsHvt7Pqv7corJf8y+NqNza2N3UgJFnsQOb1yDCRtlIvq4cOGEI0dRTeLkUgmXUACKlW5NlTFUWjvIQjONSVz2o5Lbyj+Fvk9P+iK5Ed3ct/7nxFWQVvg+YSpE2J29hW0UhmYySWkHL4QSz4iPtewZTwKpFDTYUpYewG1/WVTjSwpN03TsaqQShFSXwZlvBsEgmxhVp3FCi/PDPUDzu784EP8PX7iKKdCYaFpCf3CIUYKv0npYCV+NVK66fw5u3G48jg0TO5QmB3gkgKimeKLaG+o/Gm7DKVxvBxMnC8Nv4Xtj5exnI+IDc1iGhCCYHS4S09bNKZwL1qbP724OIyio0Jg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 9, 2023 at 6:03 PM H.J. Lu wrote: > > On Thu, Mar 9, 2023 at 5:13 PM Edgecombe, Rick P > wrote: > > > > +Joao regarding mixed mode designs > > > > On Fri, 2023-03-10 at 00:51 +0100, Borislav Petkov wrote: > > > On Thu, Mar 09, 2023 at 04:56:37PM +0000, Edgecombe, Rick P wrote: > > > > There is a proc that shows if shadow stack is enabled in a thread. > > > > It > > > > does indeed come later in the series. > > > > > > Not good enough: > > > > > > 1. buried somewhere in proc where no one knows about it > > > > > > 2. it is per thread so user needs to grep *all* > > > > See "x86: Expose thread features in /proc/$PID/status" for the patch. > > We could emit something in dmesg I guess? The logic would be: > > - Record the presence of elf SHSTK bit on exec > > - On shadow stack disable, if it had the elf bit, pr_info("bad!") > > > > > > > > > ... We previously tried to add some batch operations to improve > > > > the > > > > performance, but tglx had suggested to start with something > > > > simple. > > > > So we end up with this simple composable API. > > > > > > I agree with starting simple and thanks for explaining this in > > > detail. > > > > > > TBH, though, it already sounds like a mess to me. I guess a mess > > > we'll > > > have to deal with because there will always be this case of some > > > shared object/lib not being enabled for shstk because of raisins. > > > > The compatibility problems are totally the mess in this whole thing. > > When you try to look at a "permissive" mode that actually works it gets > > even more complex. Joao and I have been banging our heads on that > > problem for months. > > > > But there are some expected users of this that say: we compile and > > check our known set of binaries, we won't get any surprises. So it's > > more of a distro problem. > > > > > > > > And TBH #2, I would've done it even simpler: if some shared object > > > can't > > > do shadow stack, we disable it for the whole process. I mean, what's > > > the > > > point? > > > > You mean a late loaded dlopen()ed DSO? The enabling logic can't know > > this will happen ahead of time. > > > > If you mean if the shared objects in the elf all support shadow stack, > > then this is what happens. The complication is that the loader wants to > > enable shadow stack before it has checked the elf libs so it doesn't > > underflow the shadow stack when it returns from the function that does > > this checking. > > > > So it does: > > 1. Enable shadow stack > > 2. Call elf libs checking functions > > 3. If all good, lock shadow stack. Else, disable shadow stack. > > 4. Return from elf checking functions and if shstk is enabled, don't > > underflow because it was enabled in step 1 and we have return addresses > > from 2 on the shadow stack > > > > I'm wondering if this can't be improved in glibc to look like: > > 1. Check elf libs, and record it somewhere > > 2. Wait until just the right spot > > 3. If all good, enable and lock shadow stack. > > I will try it out. > Currently glibc enables shadow stack as early as possible. There are only a few places where a function call in glibc never returns. We can enable shadow stack just before calling main. There are quite some code paths without shadow stack protection. Is this an issue? H.J.