From: Szabolcs Nagy <szabolcs.nagy@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com>, linux-arch@vger.kernel.org, Peter Collingbourne <pcc@google.com>, Andrey Konovalov <andreyknvl@google.com>, Kevin Brodsky <kevin.brodsky@arm.com>, linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Will Deacon <will@kernel.org>, linux-arm-kernel@lists.infradead.org, nd@arm.com Subject: Re: [PATCH v7 29/29] arm64: mte: Add Memory Tagging Extension documentation Date: Tue, 28 Jul 2020 15:53:51 +0100 [thread overview] Message-ID: <20200728145350.GR7127@arm.com> (raw) In-Reply-To: <20200728110758.GA21941@arm.com> The 07/28/2020 12:08, Dave Martin wrote: > On Mon, Jul 27, 2020 at 05:36:35PM +0100, Szabolcs Nagy wrote: > > The 07/15/2020 18:08, Catalin Marinas wrote: > > > +The user can select the above modes, per thread, using the > > > +``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where > > > +``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK`` > > > +bit-field: > > > + > > > +- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults > > > +- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode > > > +- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode > > > + > > > +The current tag check fault mode can be read using the > > > +``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call. > > > > we discussed the need for per process prctl off list, i will > > try to summarize the requirement here: > > > > - it cannot be guaranteed in general that a library initializer > > or first call into a library happens when the process is still > > single threaded. > > > > - user code currently has no way to call prctl in all threads of > > a process and even within the c runtime doing so is problematic > > (it has to signal all threads, which requires a reserved signal > > and dealing with exiting threads and signal masks, such mechanism > > can break qemu user and various other userspace tooling). > > When working on the SVE support, I came to the conclusion that this > kind of thing would normally either be done by the runtime itself, or in > close cooperation with the runtime. However, for SVE it never makes > sense for one thread to asynchronously change the vector length of > another thread -- that's different from the MTE situation. currently there is libc mechanism to do some operation in all threads (e.g. for set*id) but this is fragile and not something that can be exposed to user code. (on the kernel side it should be much simpler to do) > > - we don't yet have defined contract in userspace about how user > > code may enable mte (i.e. use the prctl call), but it seems that > > there will be use cases for it: LD_PRELOADing malloc for heap > > tagging is one such case, but any library or custom allocator > > that wants to use mte will have this issue: when it enables mte > > it wants to enable it for all threads in the process. (or at > > least all threads managed by the c runtime). > > What are the situations where we anticipate a need to twiddle MTE in > multiple threads simultaneously, other than during process startup? > > > - even if user code is not allowed to call the prctl directly, > > i.e. the prctl settings are owned by the libc, there will be > > cases when the settings have to be changed in a multithreaded > > process (e.g. dlopening a library that requires a particular > > mte state). > > Could be avoided by refusing to dlopen a library that is incompatible > with the current process. > > dlopen()ing a library that doesn't support tagged addresses, in a > process that does use tagged addresses, seems undesirable even if tag > checking is currently turned off. yes but it can go the other way too: at startup the libc does not enable tag checks for performance reasons, but at dlopen time a library is detected to use mte (e.g. stack tagging or custom allocator). then libc or the dlopened library has to ensure that checks are enabled in all threads. (in case of stack tagging the libc has to mark existing stacks with PROT_MTE too, there is mechanism for this in glibc to deal with dlopened libraries that require executable stack and only reject the dlopen if this cannot be performed.) another usecase is that the libc is mte-safe (it accepts tagged pointers and memory in its interfaces), but it does not enable mte (this will be the case with glibc 2.32) and user libraries have to enable mte to use it (custom allocator or malloc interposition are examples). and i think this is necessary if userpsace wants to turn async tag check into sync tag check at runtime when a failure is detected. > > a solution is to introduce a flag like SECCOMP_FILTER_FLAG_TSYNC > > that means the prctl is for all threads in the process not just > > for the current one. however the exact semantics is not obvious > > if there are inconsistent settings in different threads or user > > code tries to use the prctl concurrently: first checking then > > setting the mte state via separate prctl calls is racy. but if > > the userspace contract for enabling mte limits who and when can > > call the prctl then i think the simple sync flag approach works. > > > > (the sync flag should apply to all prctl settings: tagged addr > > syscall abi, mte check fault mode, irg tag excludes. ideally it > > would work for getting the process wide state and it would fail > > in case of inconsistent settings.) > > If going down this route, perhaps we could have sets of settings: > so for each setting we have a process-wide value and a per-thread > value, with defines rules about how they combine. > > Since MTE is a debugging feature, we might be able to be less aggressive > about synchronisation than in the SECCOMP case. separate process-wide and per-thread value works for me and i expect most uses will be process wide settings. i don't think mte is less of a security feature than seccomp. if linux does not want to add a per process setting then only libc will be able to opt-in to mte and only at very early in the startup process (before executing any user code that may start threads). this is not out of question, but i think it limits the usage and deployment options. > > we may need to document some memory ordering details when > > memory accesses in other threads are affected, but i think > > that can be something simple that leaves it unspecified > > what happens with memory accesses that are not synchrnized > > with the prctl call. > > Hmmm... e.g. it may be enough if the spec only works if there is no PROT_MTE memory mapped yet, and no tagged addresses are present in the multi-threaded process when the prctl is called.
WARNING: multiple messages have this Message-ID (diff)
From: Szabolcs Nagy <szabolcs.nagy@arm.com> To: Dave Martin <Dave.Martin@arm.com> Cc: linux-arch@vger.kernel.org, nd@arm.com, Will Deacon <will@kernel.org>, Catalin Marinas <catalin.marinas@arm.com>, Kevin Brodsky <kevin.brodsky@arm.com>, linux-mm@kvack.org, Andrey Konovalov <andreyknvl@google.com>, Andrew Morton <akpm@linux-foundation.org>, Vincenzo Frascino <vincenzo.frascino@arm.com>, Peter Collingbourne <pcc@google.com>, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v7 29/29] arm64: mte: Add Memory Tagging Extension documentation Date: Tue, 28 Jul 2020 15:53:51 +0100 [thread overview] Message-ID: <20200728145350.GR7127@arm.com> (raw) In-Reply-To: <20200728110758.GA21941@arm.com> The 07/28/2020 12:08, Dave Martin wrote: > On Mon, Jul 27, 2020 at 05:36:35PM +0100, Szabolcs Nagy wrote: > > The 07/15/2020 18:08, Catalin Marinas wrote: > > > +The user can select the above modes, per thread, using the > > > +``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where > > > +``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK`` > > > +bit-field: > > > + > > > +- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults > > > +- ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode > > > +- ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode > > > + > > > +The current tag check fault mode can be read using the > > > +``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call. > > > > we discussed the need for per process prctl off list, i will > > try to summarize the requirement here: > > > > - it cannot be guaranteed in general that a library initializer > > or first call into a library happens when the process is still > > single threaded. > > > > - user code currently has no way to call prctl in all threads of > > a process and even within the c runtime doing so is problematic > > (it has to signal all threads, which requires a reserved signal > > and dealing with exiting threads and signal masks, such mechanism > > can break qemu user and various other userspace tooling). > > When working on the SVE support, I came to the conclusion that this > kind of thing would normally either be done by the runtime itself, or in > close cooperation with the runtime. However, for SVE it never makes > sense for one thread to asynchronously change the vector length of > another thread -- that's different from the MTE situation. currently there is libc mechanism to do some operation in all threads (e.g. for set*id) but this is fragile and not something that can be exposed to user code. (on the kernel side it should be much simpler to do) > > - we don't yet have defined contract in userspace about how user > > code may enable mte (i.e. use the prctl call), but it seems that > > there will be use cases for it: LD_PRELOADing malloc for heap > > tagging is one such case, but any library or custom allocator > > that wants to use mte will have this issue: when it enables mte > > it wants to enable it for all threads in the process. (or at > > least all threads managed by the c runtime). > > What are the situations where we anticipate a need to twiddle MTE in > multiple threads simultaneously, other than during process startup? > > > - even if user code is not allowed to call the prctl directly, > > i.e. the prctl settings are owned by the libc, there will be > > cases when the settings have to be changed in a multithreaded > > process (e.g. dlopening a library that requires a particular > > mte state). > > Could be avoided by refusing to dlopen a library that is incompatible > with the current process. > > dlopen()ing a library that doesn't support tagged addresses, in a > process that does use tagged addresses, seems undesirable even if tag > checking is currently turned off. yes but it can go the other way too: at startup the libc does not enable tag checks for performance reasons, but at dlopen time a library is detected to use mte (e.g. stack tagging or custom allocator). then libc or the dlopened library has to ensure that checks are enabled in all threads. (in case of stack tagging the libc has to mark existing stacks with PROT_MTE too, there is mechanism for this in glibc to deal with dlopened libraries that require executable stack and only reject the dlopen if this cannot be performed.) another usecase is that the libc is mte-safe (it accepts tagged pointers and memory in its interfaces), but it does not enable mte (this will be the case with glibc 2.32) and user libraries have to enable mte to use it (custom allocator or malloc interposition are examples). and i think this is necessary if userpsace wants to turn async tag check into sync tag check at runtime when a failure is detected. > > a solution is to introduce a flag like SECCOMP_FILTER_FLAG_TSYNC > > that means the prctl is for all threads in the process not just > > for the current one. however the exact semantics is not obvious > > if there are inconsistent settings in different threads or user > > code tries to use the prctl concurrently: first checking then > > setting the mte state via separate prctl calls is racy. but if > > the userspace contract for enabling mte limits who and when can > > call the prctl then i think the simple sync flag approach works. > > > > (the sync flag should apply to all prctl settings: tagged addr > > syscall abi, mte check fault mode, irg tag excludes. ideally it > > would work for getting the process wide state and it would fail > > in case of inconsistent settings.) > > If going down this route, perhaps we could have sets of settings: > so for each setting we have a process-wide value and a per-thread > value, with defines rules about how they combine. > > Since MTE is a debugging feature, we might be able to be less aggressive > about synchronisation than in the SECCOMP case. separate process-wide and per-thread value works for me and i expect most uses will be process wide settings. i don't think mte is less of a security feature than seccomp. if linux does not want to add a per process setting then only libc will be able to opt-in to mte and only at very early in the startup process (before executing any user code that may start threads). this is not out of question, but i think it limits the usage and deployment options. > > we may need to document some memory ordering details when > > memory accesses in other threads are affected, but i think > > that can be something simple that leaves it unspecified > > what happens with memory accesses that are not synchrnized > > with the prctl call. > > Hmmm... e.g. it may be enough if the spec only works if there is no PROT_MTE memory mapped yet, and no tagged addresses are present in the multi-threaded process when the prctl is called. _______________________________________________ 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:[~2020-07-28 14:54 UTC|newest] Thread overview: 76+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-07-15 17:08 [PATCH v7 00/26] arm64: Memory Tagging Extension user-space support Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 01/29] arm64: mte: system register definitions Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 02/29] arm64: mte: CPU feature detection and initial sysreg configuration Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 03/29] arm64: mte: Use Normal Tagged attributes for the linear map Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 04/29] arm64: mte: Add specific SIGSEGV codes Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 05/29] arm64: mte: Handle synchronous and asynchronous tag check faults Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 06/29] mm: Add PG_arch_2 page flag Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 07/29] mm: Preserve the PG_arch_2 flag in __split_huge_page_tail() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 08/29] arm64: mte: Clear the tags when a page is mapped in user-space with PROT_MTE Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 09/29] arm64: mte: Tags-aware copy_{user_,}highpage() implementations Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 10/29] arm64: Avoid unnecessary clear_user_page() indirection Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 11/29] arm64: mte: Tags-aware aware memcmp_pages() implementation Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 12/29] arm64: mte: Handle the MAIR_EL1 changes for late CPU bring-up Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 13/29] mm: Introduce arch_calc_vm_flag_bits() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 14/29] arm64: mte: Add PROT_MTE support to mmap() and mprotect() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 15/29] mm: Introduce arch_validate_flags() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 16/29] arm64: mte: Validate the PROT_MTE request via arch_validate_flags() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 17/29] mm: Allow arm64 mmap(PROT_MTE) on RAM-based files Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 18/29] arm64: mte: Allow user control of the tag check mode via prctl() Catalin Marinas 2020-07-20 15:30 ` Kevin Brodsky 2020-07-20 15:30 ` Kevin Brodsky 2020-07-20 17:00 ` Dave Martin 2020-07-20 17:00 ` Dave Martin 2020-07-22 10:28 ` Catalin Marinas 2020-07-22 10:28 ` Catalin Marinas 2020-07-23 19:33 ` Kevin Brodsky 2020-07-23 19:33 ` Kevin Brodsky 2020-07-22 11:09 ` Catalin Marinas 2020-07-22 11:09 ` Catalin Marinas 2020-08-04 19:34 ` Kevin Brodsky 2020-08-04 19:34 ` Kevin Brodsky 2020-08-05 9:24 ` Catalin Marinas 2020-08-05 9:24 ` Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 19/29] arm64: mte: Allow user control of the generated random tags " Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 20/29] arm64: mte: Restore the GCR_EL1 register after a suspend Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 21/29] arm64: mte: Allow {set,get}_tagged_addr_ctrl() on non-current tasks Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 22/29] arm64: mte: ptrace: Add PTRACE_{PEEK,POKE}MTETAGS support Catalin Marinas 2020-08-13 14:01 ` Luis Machado 2020-08-13 14:01 ` Luis Machado 2020-08-22 10:56 ` Catalin Marinas 2020-08-22 10:56 ` Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 23/29] arm64: mte: ptrace: Add NT_ARM_TAGGED_ADDR_CTRL regset Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 24/29] fs: Handle intra-page faults in copy_mount_options() Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 25/29] mm: Add arch hooks for saving/restoring tags Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 26/29] arm64: mte: Enable swap of tagged pages Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 27/29] arm64: mte: Save tags when hibernating Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 28/29] arm64: mte: Kconfig entry Catalin Marinas 2020-07-15 17:08 ` [PATCH v7 29/29] arm64: mte: Add Memory Tagging Extension documentation Catalin Marinas 2020-07-27 16:36 ` Szabolcs Nagy 2020-07-27 16:36 ` Szabolcs Nagy 2020-07-28 11:08 ` Dave Martin 2020-07-28 11:08 ` Dave Martin 2020-07-28 14:53 ` Szabolcs Nagy [this message] 2020-07-28 14:53 ` Szabolcs Nagy 2020-07-28 19:59 ` Catalin Marinas 2020-07-28 19:59 ` Catalin Marinas 2020-08-03 12:43 ` Szabolcs Nagy 2020-08-03 12:43 ` Szabolcs Nagy 2020-08-07 15:19 ` Catalin Marinas 2020-08-07 15:19 ` Catalin Marinas 2020-08-10 14:13 ` Szabolcs Nagy 2020-08-10 14:13 ` Szabolcs Nagy 2020-08-11 17:20 ` Catalin Marinas 2020-08-11 17:20 ` Catalin Marinas 2020-08-12 12:45 ` Szabolcs Nagy 2020-08-12 12:45 ` Szabolcs Nagy 2020-08-19 9:54 ` Catalin Marinas 2020-08-19 9:54 ` Catalin Marinas 2020-08-20 16:43 ` Szabolcs Nagy 2020-08-20 16:43 ` Szabolcs Nagy 2020-08-20 17:27 ` Paul Eggert 2020-08-20 17:27 ` Paul Eggert 2020-08-22 11:31 ` Catalin Marinas 2020-08-22 11:31 ` Catalin Marinas 2020-08-22 11:28 ` Catalin Marinas 2020-08-22 11:28 ` Catalin Marinas
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