From: David Hildenbrand david@redhat.com Subject: proc/vmcore: fix possible deadlock on concurrent mmap and read

Lockdep noticed that there is chance for a deadlock if we have concurrent mmap, concurrent read, and the addition/removal of a callback.

As nicely explained by Boqun:

" Lockdep warned about the above sequences because rw_semaphore is a fair read-write lock, and the following can cause a deadlock:

TASK 1 TASK 2 TASK 3 ====== ====== ====== down_write(mmap_lock); down_read(vmcore_cb_rwsem) down_write(vmcore_cb_rwsem); // blocked down_read(vmcore_cb_rwsem); // cannot get the lock because of the fairness down_read(mmap_lock); // blocked

IOW, a reader can block another read if there is a writer queued by the second reader and the lock is fair. "

To fix, convert to srcu to make this deadlock impossible. We need srcu as our callbacks can sleep. With this change, I cannot trigger any lockdep warnings.

[ 6.386519] ====================================================== [ 6.387203] WARNING: possible circular locking dependency detected [ 6.387965] 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1 Not tainted [ 6.388899] ------------------------------------------------------ [ 6.389657] makedumpfile/542 is trying to acquire lock: [ 6.390308] ffffffff832d2eb8 (vmcore_cb_rwsem){.+.+}-{3:3}, at: mmap_vmcore+0x340/0x580 [ 6.391290] [ 6.391290] but task is already holding lock: [ 6.391978] ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150 [ 6.392898] [ 6.392898] which lock already depends on the new lock. [ 6.392898] [ 6.393866] [ 6.393866] the existing dependency chain (in reverse order) is: [ 6.394762] [ 6.394762] -> #1 (&mm->mmap_lock#2){++++}-{3:3}: [ 6.395530] lock_acquire+0xc3/0x1a0 [ 6.396047] __might_fault+0x4e/0x70 [ 6.396562] _copy_to_user+0x1f/0x90 [ 6.397093] __copy_oldmem_page+0x72/0xc0 [ 6.397663] read_from_oldmem+0x77/0x1e0 [ 6.398229] read_vmcore+0x2c2/0x310 [ 6.398742] proc_reg_read+0x47/0xa0 [ 6.399265] vfs_read+0x101/0x340 [ 6.399751] __x64_sys_pread64+0x5d/0xa0 [ 6.400314] do_syscall_64+0x43/0x90 [ 6.400778] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 6.401390] [ 6.401390] -> #0 (vmcore_cb_rwsem){.+.+}-{3:3}: [ 6.402063] validate_chain+0x9f4/0x2670 [ 6.402560] __lock_acquire+0x8f7/0xbc0 [ 6.403054] lock_acquire+0xc3/0x1a0 [ 6.403509] down_read+0x4a/0x140 [ 6.403948] mmap_vmcore+0x340/0x580 [ 6.404403] proc_reg_mmap+0x3e/0x90 [ 6.404866] mmap_region+0x504/0x880 [ 6.405322] do_mmap+0x38a/0x520 [ 6.405744] vm_mmap_pgoff+0xc1/0x150 [ 6.406258] ksys_mmap_pgoff+0x178/0x200 [ 6.406823] do_syscall_64+0x43/0x90 [ 6.407339] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 6.407975] [ 6.407975] other info that might help us debug this: [ 6.407975] [ 6.408945] Possible unsafe locking scenario: [ 6.408945] [ 6.409684] CPU0 CPU1 [ 6.410196] ---- ---- [ 6.410703] lock(&mm->mmap_lock#2); [ 6.411121] lock(vmcore_cb_rwsem); [ 6.411792] lock(&mm->mmap_lock#2); [ 6.412465] lock(vmcore_cb_rwsem); [ 6.412873] [ 6.412873] *** DEADLOCK *** [ 6.412873] [ 6.413522] 1 lock held by makedumpfile/542: [ 6.414006] #0: ffff8880af226438 (&mm->mmap_lock#2){++++}-{3:3}, at: vm_mmap_pgoff+0x84/0x150 [ 6.414944] [ 6.414944] stack backtrace: [ 6.415432] CPU: 0 PID: 542 Comm: makedumpfile Not tainted 5.17.0-0.rc0.20220117git0c947b893d69.68.test.fc36.x86_64 #1 [ 6.416581] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011 [ 6.417272] Call Trace: [ 6.417593] <TASK> [ 6.417882] dump_stack_lvl+0x5d/0x78 [ 6.418346] print_circular_bug+0x5d7/0x5f0 [ 6.418821] ? stack_trace_save+0x3a/0x50 [ 6.419273] ? save_trace+0x3d/0x330 [ 6.419681] check_noncircular+0xd1/0xe0 [ 6.420217] validate_chain+0x9f4/0x2670 [ 6.420715] ? __lock_acquire+0x8f7/0xbc0 [ 6.421234] ? __lock_acquire+0x8f7/0xbc0 [ 6.421685] __lock_acquire+0x8f7/0xbc0 [ 6.422127] lock_acquire+0xc3/0x1a0 [ 6.422535] ? mmap_vmcore+0x340/0x580 [ 6.422965] ? lock_is_held_type+0xe2/0x140 [ 6.423432] ? mmap_vmcore+0x340/0x580 [ 6.423893] down_read+0x4a/0x140 [ 6.424321] ? mmap_vmcore+0x340/0x580 [ 6.424800] mmap_vmcore+0x340/0x580 [ 6.425237] ? vm_area_alloc+0x1c/0x60 [ 6.425661] ? trace_kmem_cache_alloc+0x30/0xe0 [ 6.426174] ? kmem_cache_alloc+0x1e0/0x2f0 [ 6.426641] proc_reg_mmap+0x3e/0x90 [ 6.427052] mmap_region+0x504/0x880 [ 6.427462] do_mmap+0x38a/0x520 [ 6.427842] vm_mmap_pgoff+0xc1/0x150 [ 6.428260] ksys_mmap_pgoff+0x178/0x200 [ 6.428701] do_syscall_64+0x43/0x90 [ 6.429126] entry_SYSCALL_64_after_hwframe+0x44/0xae [ 6.429745] RIP: 0033:0x7fc7359b8fc7 [ 6.430157] Code: 00 00 00 89 ef e8 69 b3 ff ff eb e4 e8 c2 64 01 00 66 90 f3 0f 1e fa 41 89 ca 41 f7 c1 ff 0f 00 00 75 10 b8 09 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 21 c3 48 8b 05 21 7e 0e 00 64 c7 00 16 00 00 [ 6.432147] RSP: 002b:00007fff35b4c208 EFLAGS: 00000246 ORIG_RAX: 0000000000000009 [ 6.432970] RAX: ffffffffffffffda RBX: 0000000000000001 RCX: 00007fc7359b8fc7 [ 6.433746] RDX: 0000000000000001 RSI: 0000000000400000 RDI: 0000000000000000 [ 6.434529] RBP: 000055a1125ecf10 R08: 0000000000000003 R09: 0000000000002000 [ 6.435310] R10: 0000000000000002 R11: 0000000000000246 R12: 0000000000002000 [ 6.436093] R13: 0000000000400000 R14: 000055a1124269e2 R15: 0000000000000000 [ 6.436887] </TASK>

Link: https://lkml.kernel.org/r/20220119193417.100385-1-david@redhat.com Fixes: cc5f2704c934 ("proc/vmcore: convert oldmem_pfn_is_ram callback to more generic vmcore callbacks") Signed-off-by: David Hildenbrand david@redhat.com Reported-by: Baoquan He bhe@redhat.com Acked-by: Baoquan He bhe@redhat.com Cc: Vivek Goyal vgoyal@redhat.com Cc: Dave Young dyoung@redhat.com Cc: "Paul E. McKenney" paulmck@kernel.org Cc: Josh Triplett josh@joshtriplett.org Cc: Peter Zijlstra peterz@infradead.org Cc: Boqun Feng boqun.feng@gmail.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/proc/vmcore.c | 41 ++++++++++++++++++++++------------------- 1 file changed, 22 insertions(+), 19 deletions(-)

--- a/fs/proc/vmcore.c~proc-vmcore-fix-possible-deadlock-on-concurrent-mmap-and-read +++ a/fs/proc/vmcore.c @@ -62,7 +62,8 @@ core_param(novmcoredd, vmcoredd_disabled /* Device Dump Size */ static size_t vmcoredd_orig_sz;

-static DECLARE_RWSEM(vmcore_cb_rwsem); +static DEFINE_SPINLOCK(vmcore_cb_lock); +DEFINE_STATIC_SRCU(vmcore_cb_srcu); /* List of registered vmcore callbacks. */ static LIST_HEAD(vmcore_cb_list); /* Whether the vmcore has been opened once. */ @@ -70,8 +71,8 @@ static bool vmcore_opened;

void register_vmcore_cb(struct vmcore_cb *cb) { - down_write(&vmcore_cb_rwsem); INIT_LIST_HEAD(&cb->next); + spin_lock(&vmcore_cb_lock); list_add_tail(&cb->next, &vmcore_cb_list); /* * Registering a vmcore callback after the vmcore was opened is @@ -79,14 +80,14 @@ void register_vmcore_cb(struct vmcore_cb */ if (vmcore_opened) pr_warn_once("Unexpected vmcore callback registration\n"); - up_write(&vmcore_cb_rwsem); + spin_unlock(&vmcore_cb_lock); } EXPORT_SYMBOL_GPL(register_vmcore_cb);

void unregister_vmcore_cb(struct vmcore_cb *cb) { - down_write(&vmcore_cb_rwsem); - list_del(&cb->next); + spin_lock(&vmcore_cb_lock); + list_del_rcu(&cb->next); /* * Unregistering a vmcore callback after the vmcore was opened is * very unusual (e.g., forced driver removal), but we cannot stop @@ -94,7 +95,9 @@ void unregister_vmcore_cb(struct vmcore_ */ if (vmcore_opened) pr_warn_once("Unexpected vmcore callback unregistration\n"); - up_write(&vmcore_cb_rwsem); + spin_unlock(&vmcore_cb_lock); + + synchronize_srcu(&vmcore_cb_srcu); } EXPORT_SYMBOL_GPL(unregister_vmcore_cb);

@@ -103,9 +106,8 @@ static bool pfn_is_ram(unsigned long pfn struct vmcore_cb *cb; bool ret = true;

- lockdep_assert_held_read(&vmcore_cb_rwsem); - - list_for_each_entry(cb, &vmcore_cb_list, next) { + list_for_each_entry_srcu(cb, &vmcore_cb_list, next, + srcu_read_lock_held(&vmcore_cb_srcu)) { if (unlikely(!cb->pfn_is_ram)) continue; ret = cb->pfn_is_ram(cb, pfn); @@ -118,9 +120,9 @@ static bool pfn_is_ram(unsigned long pfn

static int open_vmcore(struct inode *inode, struct file *file) { - down_read(&vmcore_cb_rwsem); + spin_lock(&vmcore_cb_lock); vmcore_opened = true; - up_read(&vmcore_cb_rwsem); + spin_unlock(&vmcore_cb_lock);

return 0; } @@ -133,6 +135,7 @@ ssize_t read_from_oldmem(char *buf, size unsigned long pfn, offset; size_t nr_bytes; ssize_t read = 0, tmp; + int idx;

if (!count) return 0; @@ -140,7 +143,7 @@ ssize_t read_from_oldmem(char *buf, size offset = (unsigned long)(*ppos % PAGE_SIZE); pfn = (unsigned long)(*ppos / PAGE_SIZE);

- down_read(&vmcore_cb_rwsem); + idx = srcu_read_lock(&vmcore_cb_srcu); do { if (count > (PAGE_SIZE - offset)) nr_bytes = PAGE_SIZE - offset; @@ -165,7 +168,7 @@ ssize_t read_from_oldmem(char *buf, size offset, userbuf); } if (tmp < 0) { - up_read(&vmcore_cb_rwsem); + srcu_read_unlock(&vmcore_cb_srcu, idx); return tmp; }

@@ -176,8 +179,8 @@ ssize_t read_from_oldmem(char *buf, size ++pfn; offset = 0; } while (count); + srcu_read_unlock(&vmcore_cb_srcu, idx);

- up_read(&vmcore_cb_rwsem); return read; }

@@ -568,18 +571,18 @@ static int vmcore_remap_oldmem_pfn(struc unsigned long from, unsigned long pfn, unsigned long size, pgprot_t prot) { - int ret; + int ret, idx;

/* - * Check if oldmem_pfn_is_ram was registered to avoid - * looping over all pages without a reason. + * Check if a callback was registered to avoid looping over all + * pages without a reason. */ - down_read(&vmcore_cb_rwsem); + idx = srcu_read_lock(&vmcore_cb_srcu); if (!list_empty(&vmcore_cb_list)) ret = remap_oldmem_pfn_checked(vma, from, pfn, size, prot); else ret = remap_oldmem_pfn_range(vma, from, pfn, size, prot); - up_read(&vmcore_cb_rwsem); + srcu_read_unlock(&vmcore_cb_srcu, idx); return ret; }

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