From: Catalin Marinas <catalin.marinas@arm.com> To: Szabolcs Nagy <szabolcs.nagy@arm.com> Cc: libc-alpha@sourceware.org, Mark Rutland <mark.rutland@arm.com>, Will Deacon <will@kernel.org>, linux-kernel@vger.kernel.org, Jeremy Linton <jeremy.linton@arm.com>, Mark Brown <broonie@kernel.org>, kernel-hardening@lists.openwall.com, Topi Miettinen <toiwoton@gmail.com>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831] Date: Thu, 3 Dec 2020 17:30:06 +0000 [thread overview] Message-ID: <20201203173006.GH2830@gaia> (raw) In-Reply-To: <cover.1606319495.git.szabolcs.nagy@arm.com> Hi Szabolcs, On Fri, Nov 27, 2020 at 01:19:16PM +0000, Szabolcs Nagy wrote: > This is v2 of > https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html > > To enable BTI support, re-mmap executable segments instead of > mprotecting them in case mprotect is seccomp filtered. > > I would like linux to change to map the main exe with PROT_BTI when > that is marked as BTI compatible. From the linux side i heard the > following concerns about this: > - it's an ABI change so requires some ABI bump. (this is fine with > me, i think glibc does not care about backward compat as nothing > can reasonably rely on the current behaviour, but if we have a > new bit in auxv or similar then we can save one mprotect call.) I'm not concerned about the ABI change but there are workarounds like a new auxv bit. > - in case we discover compatibility issues with user binaries it's > better if userspace can easily disable BTI (e.g. removing the > mprotect based on some env var, but if kernel adds PROT_BTI and > mprotect is filtered then we have no reliable way to remove that > from executables. this problem already exists for static linked > exes, although admittedly those are less of a compat concern.) This is our main concern. For static binaries, the linker could detect, in theory, potential issues when linking and not set the corresponding ELF information. At runtime, a dynamic linker could detect issues and avoid enabling BTI. In both cases, it's a (static or dynamic) linker decision that belongs in user-space. > - ideally PROT_BTI would be added via a new syscall that does not > interfere with PROT_EXEC filtering. (this does not conflict with > the current patches: even with a new syscall we need a fallback.) This can be discussed as a long term solution. > - solve it in systemd (e.g. turn off the filter, use better filter): > i would prefer not to have aarch64 (or BTI) specific policy in > user code. and there was no satisfying way to do this portably. I agree. I think the best for now (as a back-portable glibc fix) is to ignore the mprotect(PROT_EXEC|PROT_BTI) error that the dynamic loader gets. BTI will be disabled if MDWX is enabled. In the meantime, we should start (continue) looking at a solution that works for both systemd and the kernel and be generic enough for other architectures. The stateless nature of the current SECCOMP approach is not suitable for this W^X policy. Kees had some suggestions here but the thread seems to have died: https://lore.kernel.org/kernel-hardening/202010221256.A4F95FD11@keescook/ -- Catalin
WARNING: multiple messages have this Message-ID (diff)
From: Catalin Marinas <catalin.marinas@arm.com> To: Szabolcs Nagy <szabolcs.nagy@arm.com> Cc: Mark Rutland <mark.rutland@arm.com>, libc-alpha@sourceware.org, kernel-hardening@lists.openwall.com, linux-kernel@vger.kernel.org, Jeremy Linton <jeremy.linton@arm.com>, Mark Brown <broonie@kernel.org>, Topi Miettinen <toiwoton@gmail.com>, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831] Date: Thu, 3 Dec 2020 17:30:06 +0000 [thread overview] Message-ID: <20201203173006.GH2830@gaia> (raw) In-Reply-To: <cover.1606319495.git.szabolcs.nagy@arm.com> Hi Szabolcs, On Fri, Nov 27, 2020 at 01:19:16PM +0000, Szabolcs Nagy wrote: > This is v2 of > https://sourceware.org/pipermail/libc-alpha/2020-November/119305.html > > To enable BTI support, re-mmap executable segments instead of > mprotecting them in case mprotect is seccomp filtered. > > I would like linux to change to map the main exe with PROT_BTI when > that is marked as BTI compatible. From the linux side i heard the > following concerns about this: > - it's an ABI change so requires some ABI bump. (this is fine with > me, i think glibc does not care about backward compat as nothing > can reasonably rely on the current behaviour, but if we have a > new bit in auxv or similar then we can save one mprotect call.) I'm not concerned about the ABI change but there are workarounds like a new auxv bit. > - in case we discover compatibility issues with user binaries it's > better if userspace can easily disable BTI (e.g. removing the > mprotect based on some env var, but if kernel adds PROT_BTI and > mprotect is filtered then we have no reliable way to remove that > from executables. this problem already exists for static linked > exes, although admittedly those are less of a compat concern.) This is our main concern. For static binaries, the linker could detect, in theory, potential issues when linking and not set the corresponding ELF information. At runtime, a dynamic linker could detect issues and avoid enabling BTI. In both cases, it's a (static or dynamic) linker decision that belongs in user-space. > - ideally PROT_BTI would be added via a new syscall that does not > interfere with PROT_EXEC filtering. (this does not conflict with > the current patches: even with a new syscall we need a fallback.) This can be discussed as a long term solution. > - solve it in systemd (e.g. turn off the filter, use better filter): > i would prefer not to have aarch64 (or BTI) specific policy in > user code. and there was no satisfying way to do this portably. I agree. I think the best for now (as a back-portable glibc fix) is to ignore the mprotect(PROT_EXEC|PROT_BTI) error that the dynamic loader gets. BTI will be disabled if MDWX is enabled. In the meantime, we should start (continue) looking at a solution that works for both systemd and the kernel and be generic enough for other architectures. The stateless nature of the current SECCOMP approach is not suitable for this W^X policy. Kees had some suggestions here but the thread seems to have died: https://lore.kernel.org/kernel-hardening/202010221256.A4F95FD11@keescook/ -- Catalin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2020-12-03 17:30 UTC|newest] Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-11-27 13:19 [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) [BZ #26831] Szabolcs Nagy 2020-11-27 13:19 ` Szabolcs Nagy 2020-11-27 13:19 ` [PATCH v2 1/6] aarch64: Fix missing BTI protection from dependencies [BZ #26926] Szabolcs Nagy 2020-11-27 13:19 ` Szabolcs Nagy 2020-12-10 17:51 ` Adhemerval Zanella 2020-12-10 17:51 ` Adhemerval Zanella 2020-12-11 15:33 ` Szabolcs Nagy 2020-12-11 15:33 ` Szabolcs Nagy 2020-11-27 13:20 ` [PATCH v2 2/6] elf: lose is closely tied to _dl_map_object_from_fd Szabolcs Nagy 2020-11-27 13:20 ` Szabolcs Nagy 2020-11-27 13:20 ` [PATCH v2 3/6] elf: Fix failure handling in _dl_map_object_from_fd Szabolcs Nagy 2020-11-27 13:20 ` Szabolcs Nagy 2020-12-10 18:25 ` Adhemerval Zanella 2020-12-10 18:25 ` Adhemerval Zanella 2020-12-11 9:32 ` Szabolcs Nagy 2020-12-11 9:32 ` Szabolcs Nagy 2020-11-27 13:20 ` [PATCH v2 4/6] elf: Move note processing after l_phdr is updated Szabolcs Nagy 2020-11-27 13:20 ` Szabolcs Nagy 2020-11-27 13:21 ` [PATCH v2 5/6] elf: Pass the fd to note processing Szabolcs Nagy 2020-11-27 13:21 ` Szabolcs Nagy 2020-12-10 18:35 ` Adhemerval Zanella 2020-12-10 18:35 ` Adhemerval Zanella 2020-11-27 13:21 ` [PATCH v2 6/6] aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831] Szabolcs Nagy 2020-11-27 13:21 ` Szabolcs Nagy 2020-12-02 8:55 ` [PATCH v3 1/2] aarch64: align address for BTI protection [BZ #26988] Szabolcs Nagy 2020-12-02 8:55 ` Szabolcs Nagy 2020-12-10 18:49 ` Adhemerval Zanella 2020-12-10 18:49 ` Adhemerval Zanella 2020-12-02 8:55 ` [PATCH v3 2/2] aarch64: Use mmap to add PROT_BTI instead of mprotect [BZ #26831] Szabolcs Nagy 2020-12-02 8:55 ` Szabolcs Nagy 2020-12-10 19:12 ` Adhemerval Zanella 2020-12-10 19:12 ` Adhemerval Zanella 2020-11-30 15:56 ` [PATCH v2 0/6] aarch64: avoid mprotect(PROT_BTI|PROT_EXEC) " Szabolcs Nagy 2020-11-30 15:56 ` Szabolcs Nagy 2020-12-03 17:30 ` Catalin Marinas [this message] 2020-12-03 17:30 ` Catalin Marinas 2020-12-07 20:03 ` Szabolcs Nagy 2020-12-07 20:03 ` Szabolcs Nagy 2020-12-11 17:46 ` Catalin Marinas 2020-12-11 17:46 ` Catalin Marinas
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