From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- fs/failfs.c | 166 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 166 insertions(+) create mode 100644 fs/failfs.c (limited to 'fs/failfs.c') diff --git a/fs/failfs.c b/fs/failfs.c new file mode 100644 index 000000000..66a36da3d --- /dev/null +++ b/fs/failfs.c @@ -0,0 +1,166 @@ +// SPDX-License-Identifier: GPL-2.0-only +/* Copyright (c) 2026 Christian Brauner */ +#include +#include +#include +#include +#include +#include + +#include "internal.h" + +static struct path failfs_root_path = {}; + +void failfs_get_root(struct path *path) +{ + *path = failfs_root_path; + path_get(path); +} + +bool failfs_mnt(const struct vfsmount *mnt) +{ + return mnt->mnt_sb == failfs_root_path.mnt->mnt_sb; +} + +static int failfs_permission(struct mnt_idmap *idmap, struct inode *inode, + int mask) +{ + return -EOPNOTSUPP; +} + +static struct dentry *failfs_lookup(struct inode *dir, struct dentry *dentry, + unsigned int flags) +{ + /* Unreachable: ->permission() already failed the walk. */ + return ERR_PTR(-EOPNOTSUPP); +} + +static int failfs_getattr(struct mnt_idmap *idmap, const struct path *path, + struct kstat *stat, u32 request_mask, + unsigned int query_flags) +{ + return -EOPNOTSUPP; +} + +static const struct inode_operations failfs_dir_inode_operations = { + .permission = failfs_permission, + .lookup = failfs_lookup, + .getattr = failfs_getattr, +}; + +static const struct file_operations failfs_dir_operations = {}; + +static int failfs_d_weak_revalidate(struct dentry *dentry, unsigned int flags) +{ + /* + * The root is only ever reached as a path-walk terminal by jumping + * to it: as "/" when it is the caller's root, or through a + * /proc//{root,cwd} magic link. ->permission() already fails + * every walk of a component, but a jump lands on the root without + * one. Refuse here too so the root cannot be pinned by an O_PATH + * open or encoded into a file handle. + */ + return -EOPNOTSUPP; +} + +static char *failfs_dname(struct dentry *dentry, char *buffer, int buflen) +{ + return dynamic_dname(buffer, buflen, "failfs:/"); +} + +static const struct dentry_operations failfs_dentry_operations = { + .d_dname = failfs_dname, + .d_weak_revalidate = failfs_d_weak_revalidate, +}; + +static int failfs_statfs(struct dentry *dentry, struct kstatfs *buf) +{ + return -EOPNOTSUPP; +} + +static const struct super_operations failfs_super_operations = { + .statfs = failfs_statfs, +}; + +static int failfs_fill_super(struct super_block *s, struct fs_context *fc) +{ + struct inode *inode; + + s->s_maxbytes = MAX_LFS_FILESIZE; + s->s_blocksize = PAGE_SIZE; + s->s_blocksize_bits = PAGE_SHIFT; + s->s_magic = FAIL_FS_MAGIC; + s->s_op = &failfs_super_operations; + s->s_export_op = NULL; + s->s_xattr = NULL; + s->s_time_gran = 1; + s->s_d_flags = 0; + + inode = new_inode(s); + if (!inode) + return -ENOMEM; + + /* failfs supports no operations... */ + inode->i_mode = S_IFDIR; + set_nlink(inode, 2); + inode->i_op = &failfs_dir_inode_operations; + inode->i_fop = &failfs_dir_operations; + simple_inode_init_ts(inode); + inode->i_ino = 1; + /* ... and is immutable. */ + inode->i_flags |= S_IMMUTABLE; + + set_default_d_op(s, &failfs_dentry_operations); + s->s_root = d_make_root(inode); + if (!s->s_root) + return -ENOMEM; + + return 0; +} + +static int failfs_get_tree(struct fs_context *fc) +{ + return get_tree_single(fc, failfs_fill_super); +} + +static const struct fs_context_operations failfs_context_ops = { + .get_tree = failfs_get_tree, +}; + +static int failfs_init_fs_context(struct fs_context *fc) +{ + fc->ops = &failfs_context_ops; + fc->global = true; + fc->sb_flags |= SB_NOUSER; + fc->s_iflags |= SB_I_NOEXEC | SB_I_NODEV; + return 0; +} + +int failfs_current_chdir(void) +{ + struct path path; + + failfs_get_root(&path); + set_fs_pwd(current->fs, &path); + path_put(&path); + return 0; +} + +static struct file_system_type failfs_fs_type = { + .name = "failfs", + .init_fs_context = failfs_init_fs_context, + .kill_sb = kill_anon_super, +}; + +void __init failfs_init(void) +{ + struct vfsmount *mnt; + + /* A single instance that is member of no mount namespace. */ + mnt = kern_mount(&failfs_fs_type); + if (IS_ERR(mnt)) + panic("VFS: Failed to create failfs"); + + failfs_root_path.mnt = mnt; + failfs_root_path.dentry = mnt->mnt_root; +} -- cgit v1.3.1