procfs: avoid fetching build ID while holding VMA lock

Fix PROCMAP_QUERY to fetch optional build ID only after dropping mmap_lock
or per-VMA lock, whichever was used to lock VMA under question, to avoid
deadlock reported by syzbot:

-> #1 (&mm->mmap_lock){++++}-{4:4}:
__might_fault+0xed/0x170
_copy_to_iter+0x118/0x1720
copy_page_to_iter+0x12d/0x1e0
filemap_read+0x720/0x10a0
blkdev_read_iter+0x2b5/0x4e0
vfs_read+0x7f4/0xae0
ksys_read+0x12a/0x250
do_syscall_64+0xcb/0xf80
entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&sb->s_type->i_mutex_key#8){++++}-{4:4}:
__lock_acquire+0x1509/0x26d0
lock_acquire+0x185/0x340
down_read+0x98/0x490
blkdev_read_iter+0x2a7/0x4e0
__kernel_read+0x39a/0xa90
freader_fetch+0x1d5/0xa80
__build_id_parse.isra.0+0xea/0x6a0
do_procmap_query+0xd75/0x1050
procfs_procmap_ioctl+0x7a/0xb0
__x64_sys_ioctl+0x18e/0x210
do_syscall_64+0xcb/0xf80
entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1
---- ----
rlock(&mm->mmap_lock);
lock(&sb->s_type->i_mutex_key#8);
lock(&mm->mmap_lock);
rlock(&sb->s_type->i_mutex_key#8);

*** DEADLOCK ***

This seems to be exacerbated (as we haven't seen these syzbot reports
before that) by the recent:

777a8560fd29 ("lib/buildid: use __kernel_read() for sleepable context")

To make this safe, we need to grab file refcount while VMA is still locked, but
other than that everything is pretty straightforward. Internal build_id_parse()
API assumes VMA is passed, but it only needs the underlying file reference, so
just add another variant build_id_parse_file() that expects file passed
directly.

[[e-mail]: fix up kerneldoc]
Link: https://lkml.kernel.org/r/[e-mail]
Fixes: ed5d583a88a9 ("fs/procfs: implement efficient VMA querying API for /proc/<pid>/maps")
Signed-off-by: Andrii Nakryiko <[e-mail]>
Reported-by: <[e-mail]>
Reviewed-by: Suren Baghdasaryan <[e-mail]>
Tested-by: Suren Baghdasaryan <[e-mail]>
Reviewed-by: Shakeel Butt <[e-mail]>
Cc: Alexei Starovoitov <[e-mail]>
Cc: Daniel Borkmann <[e-mail]>
Cc: Eduard Zingerman <[e-mail]>
Cc: Hao Luo <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: John Fastabend <[e-mail]>
Cc: KP Singh <[e-mail]>
Cc: Martin KaFai Lau <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: Stanislav Fomichev <[e-mail]>
Cc: Yonghong Song <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

fs/proc/task_mmu.c

···
 	struct proc_maps_locking_ctx lock_ctx = { .mm = mm };
 	struct procmap_query karg;
 	struct vm_area_struct *vma;
+	struct file *vm_file = NULL;
 	const char *name = NULL;
 	char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL;
 	__u64 usize;
···
 		karg.inode = 0;
 	}
 
-	if (karg.build_id_size) {
-		__u32 build_id_sz;
-
-		err = build_id_parse(vma, build_id_buf, &build_id_sz);
-		if (err) {
-			karg.build_id_size = 0;
-		} else {
-			if (karg.build_id_size < build_id_sz) {
-				err = -ENAMETOOLONG;
-				goto out;
-			}
-			karg.build_id_size = build_id_sz;
-		}
-	}
-
 	if (karg.vma_name_size) {
 		size_t name_buf_sz = min_t(size_t, PATH_MAX, karg.vma_name_size);
 		const struct path *path;
···
 		karg.vma_name_size = name_sz;
 	}
 
+	if (karg.build_id_size && vma->vm_file)
+		vm_file = get_file(vma->vm_file);
+
 	/* unlock vma or mmap_lock, and put mm_struct before copying data to user */
 	query_vma_teardown(&lock_ctx);
 	mmput(mm);
+
+	if (karg.build_id_size) {
+		__u32 build_id_sz;
+
+		if (vm_file)
+			err = build_id_parse_file(vm_file, build_id_buf, &build_id_sz);
+		else
+			err = -ENOENT;
+		if (err) {
+			karg.build_id_size = 0;
+		} else {
+			if (karg.build_id_size < build_id_sz) {
+				err = -ENAMETOOLONG;
+				goto out;
+			}
+			karg.build_id_size = build_id_sz;
+		}
+	}
+
+	if (vm_file)
+		fput(vm_file);
 
 	if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),
 					       name, karg.vma_name_size)) {
···
 out:
 	query_vma_teardown(&lock_ctx);
 	mmput(mm);
+	if (vm_file)
+		fput(vm_file);
 	kfree(name_buf);
 	return err;
 }

include/linux/buildid.h

···
 #define BUILD_ID_SIZE_MAX 20
 
 struct vm_area_struct;
+struct file;
+
 int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size);
+int build_id_parse_file(struct file *file, unsigned char *build_id, __u32 *size);
 int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size);
 int build_id_parse_buf(const void *buf, unsigned char *build_id, u32 buf_size);
 

lib/buildid.c

···
 /* enough for Elf64_Ehdr, Elf64_Phdr, and all the smaller requests */
 #define MAX_FREADER_BUF_SZ 64
 
-static int __build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
+static int __build_id_parse(struct file *file, unsigned char *build_id,
 			    __u32 *size, bool may_fault)
 {
 	const Elf32_Ehdr *ehdr;
···
 	char buf[MAX_FREADER_BUF_SZ];
 	int ret;
 
-	/* only works for page backed storage  */
-	if (!vma->vm_file)
-		return -EINVAL;
-
-	freader_init_from_file(&r, buf, sizeof(buf), vma->vm_file, may_fault);
+	freader_init_from_file(&r, buf, sizeof(buf), file, may_fault);
 
 	/* fetch first 18 bytes of ELF header for checks */
 	ehdr = freader_fetch(&r, 0, offsetofend(Elf32_Ehdr, e_type));
···
 	return ret;
 }
 
-/*
- * Parse build ID of ELF file mapped to vma
+/**
+ * build_id_parse_nofault() - Parse build ID of ELF file mapped to vma
  * @vma:      vma object
  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
  * @size:     returns actual build id size in case of success
···
  */
 int build_id_parse_nofault(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 {
-	return __build_id_parse(vma, build_id, size, false /* !may_fault */);
+	if (!vma->vm_file)
+		return -EINVAL;
+
+	return __build_id_parse(vma->vm_file, build_id, size, false /* !may_fault */);
 }
 
-/*
- * Parse build ID of ELF file mapped to VMA
+/**
+ * build_id_parse() - Parse build ID of ELF file mapped to VMA
  * @vma:      vma object
  * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
  * @size:     returns actual build id size in case of success
···
  */
 int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id, __u32 *size)
 {
-	return __build_id_parse(vma, build_id, size, true /* may_fault */);
+	if (!vma->vm_file)
+		return -EINVAL;
+
+	return __build_id_parse(vma->vm_file, build_id, size, true /* may_fault */);
+}
+
+/**
+ * build_id_parse_file() - Parse build ID of ELF file
+ * @file:      file object
+ * @build_id: buffer to store build id, at least BUILD_ID_SIZE long
+ * @size:     returns actual build id size in case of success
+ *
+ * Assumes faultable context and can cause page faults to bring in file data
+ * into page cache.
+ *
+ * Return: 0 on success; negative error, otherwise
+ */
+int build_id_parse_file(struct file *file, unsigned char *build_id, __u32 *size)
+{
+	return __build_id_parse(file, build_id, size, true /* may_fault */);
 }
 
 /**