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/mnt_idmapping.c | 389 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 389 insertions(+) create mode 100644 fs/mnt_idmapping.c (limited to 'fs/mnt_idmapping.c') diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c new file mode 100644 index 000000000..cb61fbdb5 --- /dev/null +++ b/fs/mnt_idmapping.c @@ -0,0 +1,389 @@ +// SPDX-License-Identifier: GPL-2.0 +/* Copyright (c) 2022 Christian Brauner */ + +#include +#include +#include +#include +#include +#include + +#include "internal.h" + +/* + * Outside of this file vfs{g,u}id_t are always created from k{g,u}id_t, + * never from raw values. These are just internal helpers. + */ +#define VFSUIDT_INIT_RAW(val) (vfsuid_t){ val } +#define VFSGIDT_INIT_RAW(val) (vfsgid_t){ val } + +struct mnt_idmap { + struct uid_gid_map uid_map; + struct uid_gid_map gid_map; + refcount_t count; +}; + +/* + * Carries the initial idmapping of 0:0:4294967295 which is an identity + * mapping. This means that {g,u}id 0 is mapped to {g,u}id 0, {g,u}id 1 is + * mapped to {g,u}id 1, [...], {g,u}id 1000 to {g,u}id 1000, [...]. + */ +struct mnt_idmap nop_mnt_idmap = { + .count = REFCOUNT_INIT(1), +}; +EXPORT_SYMBOL_GPL(nop_mnt_idmap); + +/* + * Carries the invalid idmapping of a full 0-4294967295 {g,u}id range. + * This means that all {g,u}ids are mapped to INVALID_VFS{G,U}ID. + */ +struct mnt_idmap invalid_mnt_idmap = { + .count = REFCOUNT_INIT(1), +}; +EXPORT_SYMBOL_GPL(invalid_mnt_idmap); + +/** + * initial_idmapping - check whether this is the initial mapping + * @ns: idmapping to check + * + * Check whether this is the initial mapping, mapping 0 to 0, 1 to 1, + * [...], 1000 to 1000 [...]. + * + * Return: true if this is the initial mapping, false if not. + */ +static inline bool initial_idmapping(const struct user_namespace *ns) +{ + return ns == &init_user_ns; +} + +/** + * make_vfsuid - map a filesystem kuid according to an idmapping + * @idmap: the mount's idmapping + * @fs_userns: the filesystem's idmapping + * @kuid : kuid to be mapped + * + * Take a @kuid and remap it from @fs_userns into @idmap. Use this + * function when preparing a @kuid to be reported to userspace. + * + * If initial_idmapping() determines that this is not an idmapped mount + * we can simply return @kuid unchanged. + * If initial_idmapping() tells us that the filesystem is not mounted with an + * idmapping we know the value of @kuid won't change when calling + * from_kuid() so we can simply retrieve the value via __kuid_val() + * directly. + * + * Return: @kuid mapped according to @idmap. + * If @kuid has no mapping in either @idmap or @fs_userns INVALID_UID is + * returned. + */ + +vfsuid_t make_vfsuid(struct mnt_idmap *idmap, + struct user_namespace *fs_userns, + kuid_t kuid) +{ + uid_t uid; + + if (idmap == &nop_mnt_idmap) + return VFSUIDT_INIT(kuid); + if (idmap == &invalid_mnt_idmap) + return INVALID_VFSUID; + if (initial_idmapping(fs_userns)) + uid = __kuid_val(kuid); + else + uid = from_kuid(fs_userns, kuid); + if (uid == (uid_t)-1) + return INVALID_VFSUID; + return VFSUIDT_INIT_RAW(map_id_down(&idmap->uid_map, uid)); +} +EXPORT_SYMBOL_GPL(make_vfsuid); + +/** + * make_vfsgid - map a filesystem kgid according to an idmapping + * @idmap: the mount's idmapping + * @fs_userns: the filesystem's idmapping + * @kgid : kgid to be mapped + * + * Take a @kgid and remap it from @fs_userns into @idmap. Use this + * function when preparing a @kgid to be reported to userspace. + * + * If initial_idmapping() determines that this is not an idmapped mount + * we can simply return @kgid unchanged. + * If initial_idmapping() tells us that the filesystem is not mounted with an + * idmapping we know the value of @kgid won't change when calling + * from_kgid() so we can simply retrieve the value via __kgid_val() + * directly. + * + * Return: @kgid mapped according to @idmap. + * If @kgid has no mapping in either @idmap or @fs_userns INVALID_GID is + * returned. + */ +vfsgid_t make_vfsgid(struct mnt_idmap *idmap, + struct user_namespace *fs_userns, kgid_t kgid) +{ + gid_t gid; + + if (idmap == &nop_mnt_idmap) + return VFSGIDT_INIT(kgid); + if (idmap == &invalid_mnt_idmap) + return INVALID_VFSGID; + if (initial_idmapping(fs_userns)) + gid = __kgid_val(kgid); + else + gid = from_kgid(fs_userns, kgid); + if (gid == (gid_t)-1) + return INVALID_VFSGID; + return VFSGIDT_INIT_RAW(map_id_down(&idmap->gid_map, gid)); +} +EXPORT_SYMBOL_GPL(make_vfsgid); + +/** + * from_vfsuid - map a vfsuid into the filesystem idmapping + * @idmap: the mount's idmapping + * @fs_userns: the filesystem's idmapping + * @vfsuid : vfsuid to be mapped + * + * Map @vfsuid into the filesystem idmapping. This function has to be used in + * order to e.g. write @vfsuid to inode->i_uid. + * + * Return: @vfsuid mapped into the filesystem idmapping + */ +kuid_t from_vfsuid(struct mnt_idmap *idmap, + struct user_namespace *fs_userns, vfsuid_t vfsuid) +{ + uid_t uid; + + if (idmap == &nop_mnt_idmap) + return AS_KUIDT(vfsuid); + if (idmap == &invalid_mnt_idmap) + return INVALID_UID; + uid = map_id_up(&idmap->uid_map, __vfsuid_val(vfsuid)); + if (uid == (uid_t)-1) + return INVALID_UID; + if (initial_idmapping(fs_userns)) + return KUIDT_INIT(uid); + return make_kuid(fs_userns, uid); +} +EXPORT_SYMBOL_GPL(from_vfsuid); + +/** + * from_vfsgid - map a vfsgid into the filesystem idmapping + * @idmap: the mount's idmapping + * @fs_userns: the filesystem's idmapping + * @vfsgid : vfsgid to be mapped + * + * Map @vfsgid into the filesystem idmapping. This function has to be used in + * order to e.g. write @vfsgid to inode->i_gid. + * + * Return: @vfsgid mapped into the filesystem idmapping + */ +kgid_t from_vfsgid(struct mnt_idmap *idmap, + struct user_namespace *fs_userns, vfsgid_t vfsgid) +{ + gid_t gid; + + if (idmap == &nop_mnt_idmap) + return AS_KGIDT(vfsgid); + if (idmap == &invalid_mnt_idmap) + return INVALID_GID; + gid = map_id_up(&idmap->gid_map, __vfsgid_val(vfsgid)); + if (gid == (gid_t)-1) + return INVALID_GID; + if (initial_idmapping(fs_userns)) + return KGIDT_INIT(gid); + return make_kgid(fs_userns, gid); +} +EXPORT_SYMBOL_GPL(from_vfsgid); + +#ifdef CONFIG_MULTIUSER +/** + * vfsgid_in_group_p() - check whether a vfsuid matches the caller's groups + * @vfsgid: the mnt gid to match + * + * This function can be used to determine whether @vfsuid matches any of the + * caller's groups. + * + * Return: 1 if vfsuid matches caller's groups, 0 if not. + */ +int vfsgid_in_group_p(vfsgid_t vfsgid) +{ + return in_group_p(AS_KGIDT(vfsgid)); +} +#else +int vfsgid_in_group_p(vfsgid_t vfsgid) +{ + return 1; +} +#endif +EXPORT_SYMBOL_GPL(vfsgid_in_group_p); + +static int copy_mnt_idmap(struct uid_gid_map *map_from, + struct uid_gid_map *map_to) +{ + struct uid_gid_extent *forward, *reverse; + u32 nr_extents = READ_ONCE(map_from->nr_extents); + /* Pairs with smp_wmb() when writing the idmapping. */ + smp_rmb(); + + /* + * Don't blindly copy @map_to into @map_from if nr_extents is + * smaller or equal to UID_GID_MAP_MAX_BASE_EXTENTS. Since we + * read @nr_extents someone could have written an idmapping and + * then we might end up with inconsistent data. So just don't do + * anything at all. + */ + if (nr_extents == 0) + return -EINVAL; + + /* + * Here we know that nr_extents is greater than zero which means + * a map has been written. Since idmappings can't be changed + * once they have been written we know that we can safely copy + * from @map_to into @map_from. + */ + + if (nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) { + *map_to = *map_from; + return 0; + } + + forward = kmemdup_array(map_from->forward, nr_extents, + sizeof(struct uid_gid_extent), + GFP_KERNEL_ACCOUNT); + if (!forward) + return -ENOMEM; + + reverse = kmemdup_array(map_from->reverse, nr_extents, + sizeof(struct uid_gid_extent), + GFP_KERNEL_ACCOUNT); + if (!reverse) { + kfree(forward); + return -ENOMEM; + } + + /* + * The idmapping isn't exposed anywhere so we don't need to care + * about ordering between extent pointers and @nr_extents + * initialization. + */ + map_to->forward = forward; + map_to->reverse = reverse; + map_to->nr_extents = nr_extents; + return 0; +} + +static void free_mnt_idmap(struct mnt_idmap *idmap) +{ + if (idmap->uid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) { + kfree(idmap->uid_map.forward); + kfree(idmap->uid_map.reverse); + } + if (idmap->gid_map.nr_extents > UID_GID_MAP_MAX_BASE_EXTENTS) { + kfree(idmap->gid_map.forward); + kfree(idmap->gid_map.reverse); + } + kfree(idmap); +} + +struct mnt_idmap *alloc_mnt_idmap(struct user_namespace *mnt_userns) +{ + struct mnt_idmap *idmap; + int ret; + + idmap = kzalloc_obj(struct mnt_idmap, GFP_KERNEL_ACCOUNT); + if (!idmap) + return ERR_PTR(-ENOMEM); + + refcount_set(&idmap->count, 1); + ret = copy_mnt_idmap(&mnt_userns->uid_map, &idmap->uid_map); + if (!ret) + ret = copy_mnt_idmap(&mnt_userns->gid_map, &idmap->gid_map); + if (ret) { + free_mnt_idmap(idmap); + idmap = ERR_PTR(ret); + } + return idmap; +} + +/** + * mnt_idmap_get - get a reference to an idmapping + * @idmap: the idmap to bump the reference on + * + * If @idmap is not the @nop_mnt_idmap bump the reference count. + * + * Return: @idmap with reference count bumped if @not_mnt_idmap isn't passed. + */ +struct mnt_idmap *mnt_idmap_get(struct mnt_idmap *idmap) +{ + if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap) + refcount_inc(&idmap->count); + + return idmap; +} +EXPORT_SYMBOL_GPL(mnt_idmap_get); + +/** + * mnt_idmap_put - put a reference to an idmapping + * @idmap: the idmap to put the reference on + * + * If this is a non-initial idmapping, put the reference count when a mount is + * released and free it if we're the last user. + */ +void mnt_idmap_put(struct mnt_idmap *idmap) +{ + if (idmap != &nop_mnt_idmap && idmap != &invalid_mnt_idmap && + refcount_dec_and_test(&idmap->count)) + free_mnt_idmap(idmap); +} +EXPORT_SYMBOL_GPL(mnt_idmap_put); + +int statmount_mnt_idmap(struct mnt_idmap *idmap, struct seq_file *seq, bool uid_map) +{ + struct uid_gid_map *map, *map_up; + u32 idx, nr_mappings; + + if (!is_valid_mnt_idmap(idmap)) + return 0; + + /* + * Idmappings are shown relative to the caller's idmapping. + * This is both the most intuitive and most useful solution. + */ + if (uid_map) { + map = &idmap->uid_map; + map_up = ¤t_user_ns()->uid_map; + } else { + map = &idmap->gid_map; + map_up = ¤t_user_ns()->gid_map; + } + + for (idx = 0, nr_mappings = 0; idx < map->nr_extents; idx++) { + uid_t lower; + struct uid_gid_extent *extent; + + if (map->nr_extents <= UID_GID_MAP_MAX_BASE_EXTENTS) + extent = &map->extent[idx]; + else + extent = &map->forward[idx]; + + /* + * Verify that the whole range of the mapping can be + * resolved in the caller's idmapping. If it cannot be + * resolved skip the mapping. + */ + lower = map_id_range_up(map_up, extent->lower_first, extent->count); + if (lower == (uid_t) -1) + continue; + + seq_printf(seq, "%u %u %u", extent->first, lower, extent->count); + if (seq_has_overflowed(seq)) + return -EAGAIN; + + seq->count++; /* mappings are separated by \0 */ + if (seq_has_overflowed(seq)) + return -EAGAIN; + + nr_mappings++; + } + + return nr_mappings; +} -- cgit v1.3.1