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Wed, 14 Oct 2020 17:00:55 -0700 (PDT) Received: from localhost ([2a02:168:96c5:1:55ed:514f:6ad7:5bcc]) by smtp.gmail.com with ESMTPSA id q8sm1428293wro.32.2020.10.14.17.00.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 14 Oct 2020 17:00:54 -0700 (PDT) From: Jann Horn To: Andrew Morton , linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, "Eric W . Biederman" , Michel Lespinasse , Mauro Carvalho Chehab , Sakari Ailus , Jeff Dike , Richard Weinberger , Anton Ivanov , linux-um@lists.infradead.org, Jason Gunthorpe , John Hubbard , Johannes Berg Subject: [PATCH v3 1/2] mmap locking API: Order lock of nascent mm outside lock of live mm Date: Thu, 15 Oct 2020 02:00:40 +0200 Message-Id: <20201015000041.1734214-2-jannh@google.com> X-Mailer: git-send-email 2.28.0.1011.ga647a8990f-goog In-Reply-To: <20201015000041.1734214-1-jannh@google.com> References: <20201015000041.1734214-1-jannh@google.com> MIME-Version: 1.0 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: Until now, the mmap lock of the nascent mm was ordered inside the mmap lock of the old mm (in dup_mmap() and in UML's activate_mm()). A following patch will change the exec path to very broadly lock the nascent mm, but fine-grained locking should still work at the same time for the old mm. In particular, mmap locking calls are hidden behind the copy_from_user() calls and such that are reached through functions like copy_strings() - when a page fault occurs on a userspace memory access, the mmap lock will be taken. To do this in a way that lockdep is happy about, let's turn around the lock ordering in both places that currently nest the locks. Since SINGLE_DEPTH_NESTING is normally used for the inner nesting layer, make up our own lock subclass MMAP_LOCK_SUBCLASS_NASCENT and use that instead. The added locking calls in exec_mmap() are temporary; the following patch will move the locking out of exec_mmap(). As Johannes Berg pointed out[1][2], moving the mmap locking of arch/um/'s activate_mm() up into the execve code also fixes an issue that would've caused a scheduling-in-atomic bug due to mmap_write_lock_nested() while holding a spinlock if UM had support for voluntary preemption. (Even when a semaphore is uncontended, locking it can still trigger rescheduling via might_sleep().) [1] https://lore.kernel.org/linux-mm/115d17aa221b73a479e26ffee52899ddb18b1f53.camel@sipsolutions.net/ [2] https://lore.kernel.org/linux-mm/7b7d6954b74e109e653539d880173fa9cb5c5ddf.camel@sipsolutions.net/ Signed-off-by: Jann Horn --- arch/um/include/asm/mmu_context.h | 3 +-- fs/exec.c | 4 ++++ include/linux/mmap_lock.h | 23 +++++++++++++++++++++-- kernel/fork.c | 7 ++----- 4 files changed, 28 insertions(+), 9 deletions(-) diff --git a/arch/um/include/asm/mmu_context.h b/arch/um/include/asm/mmu_context.h index 17ddd4edf875..c13bc5150607 100644 --- a/arch/um/include/asm/mmu_context.h +++ b/arch/um/include/asm/mmu_context.h @@ -48,9 +48,8 @@ static inline void activate_mm(struct mm_struct *old, struct mm_struct *new) * when the new ->mm is used for the first time. */ __switch_mm(&new->context.id); - mmap_write_lock_nested(new, SINGLE_DEPTH_NESTING); + mmap_assert_write_locked(new); uml_setup_stubs(new); - mmap_write_unlock(new); } static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, diff --git a/fs/exec.c b/fs/exec.c index a91003e28eaa..229dbc7aa61a 100644 --- a/fs/exec.c +++ b/fs/exec.c @@ -1114,6 +1114,8 @@ static int exec_mmap(struct mm_struct *mm) if (ret) return ret; + mmap_write_lock_nascent(mm); + if (old_mm) { /* * Make sure that if there is a core dump in progress @@ -1125,6 +1127,7 @@ static int exec_mmap(struct mm_struct *mm) if (unlikely(old_mm->core_state)) { mmap_read_unlock(old_mm); mutex_unlock(&tsk->signal->exec_update_mutex); + mmap_write_unlock(mm); return -EINTR; } } @@ -1138,6 +1141,7 @@ static int exec_mmap(struct mm_struct *mm) tsk->mm->vmacache_seqnum = 0; vmacache_flush(tsk); task_unlock(tsk); + mmap_write_unlock(mm); if (old_mm) { mmap_read_unlock(old_mm); BUG_ON(active_mm != old_mm); diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 0707671851a8..24de1fe99ee4 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -3,6 +3,18 @@ #include +/* + * Lock subclasses for the mmap_lock. + * + * MMAP_LOCK_SUBCLASS_NASCENT is for core kernel code that wants to lock an mm + * that is still being constructed and wants to be able to access the active mm + * normally at the same time. It nests outside MMAP_LOCK_SUBCLASS_NORMAL. + */ +enum { + MMAP_LOCK_SUBCLASS_NORMAL = 0, + MMAP_LOCK_SUBCLASS_NASCENT +}; + #define MMAP_LOCK_INITIALIZER(name) \ .mmap_lock = __RWSEM_INITIALIZER((name).mmap_lock), @@ -16,9 +28,16 @@ static inline void mmap_write_lock(struct mm_struct *mm) down_write(&mm->mmap_lock); } -static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass) +/* + * Lock an mm_struct that is still being set up (during fork or exec). + * This nests outside the mmap locks of live mm_struct instances. + * No interruptible/killable versions exist because at the points where you're + * supposed to use this helper, the mm isn't visible to anything else, so we + * expect the mmap_lock to be uncontended. + */ +static inline void mmap_write_lock_nascent(struct mm_struct *mm) { - down_write_nested(&mm->mmap_lock, subclass); + down_write_nested(&mm->mmap_lock, MMAP_LOCK_SUBCLASS_NASCENT); } static inline int mmap_write_lock_killable(struct mm_struct *mm) diff --git a/kernel/fork.c b/kernel/fork.c index da8d360fb032..db67eb4ac7bd 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -474,6 +474,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, unsigned long charge; LIST_HEAD(uf); + mmap_write_lock_nascent(mm); uprobe_start_dup_mmap(); if (mmap_write_lock_killable(oldmm)) { retval = -EINTR; @@ -481,10 +482,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, } flush_cache_dup_mm(oldmm); uprobe_dup_mmap(oldmm, mm); - /* - * Not linked in yet - no deadlock potential: - */ - mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING); /* No ordering required: file already has been exposed. */ RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); @@ -600,12 +597,12 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, /* a new mm has just been created */ retval = arch_dup_mmap(oldmm, mm); out: - mmap_write_unlock(mm); flush_tlb_mm(oldmm); mmap_write_unlock(oldmm); dup_userfaultfd_complete(&uf); fail_uprobe_end: uprobe_end_dup_mmap(); + mmap_write_unlock(mm); return retval; fail_nomem_anon_vma_fork: mpol_put(vma_policy(tmp));