Re: [RFC bpf-next 3/5] bpf: pin, translate, and unpin __kptr_user from syscalls.

Alexei Starovoitov
2024-08-12T16:45:49Z

On Wed, Aug 7, 2024 at 4:58 PM Kui-Feng Lee <thinker.li@gmail.com> wrote:
>
> User kptrs are pinned, by pin_user_pages_fast(), and translated to an
> address in the kernel when the value is updated by user programs. (Call
> bpf_map_update_elem() from user programs.) And, the pinned pages are
> unpinned if the value of user kptrs are overritten or if the values of maps
> are deleted/destroyed.
>
> The pages are mapped through vmap() in order to get a continuous space in
> the kernel if the memory pointed by a user kptr resides in two or more
> pages. For the case of single page, page_address() is called to get the
> address of a page in the kernel.
>
> User kptr is only supported by task storage maps.
>
> One user kptr can pin at most KPTR_USER_MAX_PAGES(16) physical pages. This
> is a random picked number for safety. We actually can remove this
> restriction totally.
>
> User kptrs could only be set by user programs through syscalls. Any
> attempts of updating the value of a map with __kptr_user in it should
> ignore the values of user kptrs from BPF programs. The values of user kptrs
> will keep as they were if the new values are from BPF programs, not from
> user programs.
>
> Cc: linux-mm@kvack.org
> Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
> ---
> include/linux/bpf.h | 35 +++++-
> include/linux/bpf_local_storage.h | 2 +-
> kernel/bpf/bpf_local_storage.c | 18 +--
> kernel/bpf/helpers.c | 12 +-
> kernel/bpf/local_storage.c | 2 +-
> kernel/bpf/syscall.c | 177 +++++++++++++++++++++++++++++-
> net/core/bpf_sk_storage.c | 2 +-
> 7 files changed, 227 insertions(+), 21 deletions(-)
>
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 87d5f98249e2..f4ad0bc183cb 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -30,6 +30,7 @@
> #include <linux/static_call.h>
> #include <linux/memcontrol.h>
> #include <linux/cfi.h>
> +#include <linux/mm.h>
>
> struct bpf_verifier_env;
> struct bpf_verifier_log;
> @@ -477,10 +478,12 @@ static inline void bpf_long_memcpy(void *dst, const void *src, u32 size)
> data_race(*ldst++ = *lsrc++);
> }
>
> +void bpf_obj_unpin_uaddr(const struct btf_field *field, void *addr);
> +
> /* copy everything but bpf_spin_lock, bpf_timer, and kptrs. There could be one of each. */
> static inline void bpf_obj_memcpy(struct btf_record *rec,
> void *dst, void *src, u32 size,
> - bool long_memcpy)
> + bool long_memcpy, bool from_user)
> {
> u32 curr_off = 0;
> int i;
> @@ -496,21 +499,40 @@ static inline void bpf_obj_memcpy(struct btf_record *rec,
> for (i = 0; i < rec->cnt; i++) {
> u32 next_off = rec->fields[i].offset;
> u32 sz = next_off - curr_off;
> + void *addr;
>
> memcpy(dst + curr_off, src + curr_off, sz);
> + if (from_user && rec->fields[i].type == BPF_KPTR_USER) {

Do not add this to bpf_obj_memcpy() which is a critical path
for various map operations.
This has to be standalone for task storage only.

> + /* Unpin old address.
> + *
> + * Alignments are guaranteed by btf_find_field_one().
> + */
> + addr = *(void **)(dst + next_off);
> + if (virt_addr_valid(addr))
> + bpf_obj_unpin_uaddr(&rec->fields[i], addr);
> + else if (addr)
> + WARN_ON_ONCE(1);
> +
> + *(void **)(dst + next_off) = *(void **)(src + next_off);
> + }
> curr_off += rec->fields[i].size + sz;
> }
> memcpy(dst + curr_off, src + curr_off, size - curr_off);
> }
>
> +static inline void copy_map_value_user(struct bpf_map *map, void *dst, void *src, bool from_user)

No need for these helpers either.

> +{
> + bpf_obj_memcpy(map->record, dst, src, map->value_size, false, from_user);
> +}
> +
> static inline void copy_map_value(struct bpf_map *map, void *dst, void *src)
> {
> - bpf_obj_memcpy(map->record, dst, src, map->value_size, false);
> + bpf_obj_memcpy(map->record, dst, src, map->value_size, false, false);
> }
>
> static inline void copy_map_value_long(struct bpf_map *map, void *dst, void *src)
> {
> - bpf_obj_memcpy(map->record, dst, src, map->value_size, true);
> + bpf_obj_memcpy(map->record, dst, src, map->value_size, true, false);
> }
>
> static inline void bpf_obj_memzero(struct btf_record *rec, void *dst, u32 size)
> @@ -538,6 +560,8 @@ static inline void zero_map_value(struct bpf_map *map, void *dst)
> bpf_obj_memzero(map->record, dst, map->value_size);
> }
>
> +void copy_map_value_locked_user(struct bpf_map *map, void *dst, void *src,
> + bool lock_src, bool from_user);
> void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
> bool lock_src);
> void bpf_timer_cancel_and_free(void *timer);
> @@ -775,6 +799,11 @@ enum bpf_arg_type {
> };
> static_assert(__BPF_ARG_TYPE_MAX <= BPF_BASE_TYPE_LIMIT);
>
> +#define BPF_MAP_UPDATE_FLAG_BITS 3
> +enum bpf_map_update_flag {
> + BPF_FROM_USER = BIT(0 + BPF_MAP_UPDATE_FLAG_BITS)
> +};
> +
> /* type of values returned from helper functions */
> enum bpf_return_type {
> RET_INTEGER, /* function returns integer */
> diff --git a/include/linux/bpf_local_storage.h b/include/linux/bpf_local_storage.h
> index dcddb0aef7d8..d337df68fa23 100644
> --- a/include/linux/bpf_local_storage.h
> +++ b/include/linux/bpf_local_storage.h
> @@ -181,7 +181,7 @@ void bpf_selem_link_map(struct bpf_local_storage_map *smap,
>
> struct bpf_local_storage_elem *
> bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner, void *value,
> - bool charge_mem, gfp_t gfp_flags);
> + bool charge_mem, gfp_t gfp_flags, bool from_user);
>
> void bpf_selem_free(struct bpf_local_storage_elem *selem,
> struct bpf_local_storage_map *smap,
> diff --git a/kernel/bpf/bpf_local_storage.c b/kernel/bpf/bpf_local_storage.c
> index c938dea5ddbf..c4cf09e27a19 100644
> --- a/kernel/bpf/bpf_local_storage.c
> +++ b/kernel/bpf/bpf_local_storage.c
> @@ -73,7 +73,7 @@ static bool selem_linked_to_map(const struct bpf_local_storage_elem *selem)
>
> struct bpf_local_storage_elem *
> bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
> - void *value, bool charge_mem, gfp_t gfp_flags)
> + void *value, bool charge_mem, gfp_t gfp_flags, bool from_user)
> {
> struct bpf_local_storage_elem *selem;
>
> @@ -100,7 +100,7 @@ bpf_selem_alloc(struct bpf_local_storage_map *smap, void *owner,
>
> if (selem) {
> if (value)
> - copy_map_value(&smap->map, SDATA(selem)->data, value);
> + copy_map_value_user(&smap->map, SDATA(selem)->data, value, from_user);
> /* No need to call check_and_init_map_value as memory is zero init */
> return selem;
> }
> @@ -530,9 +530,11 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> struct bpf_local_storage_elem *alloc_selem, *selem = NULL;
> struct bpf_local_storage *local_storage;
> unsigned long flags;
> + bool from_user = map_flags & BPF_FROM_USER;
> int err;
>
> /* BPF_EXIST and BPF_NOEXIST cannot be both set */
> + map_flags &= ~BPF_FROM_USER;
> if (unlikely((map_flags & ~BPF_F_LOCK) > BPF_EXIST) ||
> /* BPF_F_LOCK can only be used in a value with spin_lock */
> unlikely((map_flags & BPF_F_LOCK) &&
> @@ -550,7 +552,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> if (err)
> return ERR_PTR(err);
>
> - selem = bpf_selem_alloc(smap, owner, value, true, gfp_flags);
> + selem = bpf_selem_alloc(smap, owner, value, true, gfp_flags, from_user);
> if (!selem)
> return ERR_PTR(-ENOMEM);
>
> @@ -575,8 +577,8 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> if (err)
> return ERR_PTR(err);
> if (old_sdata && selem_linked_to_storage_lockless(SELEM(old_sdata))) {
> - copy_map_value_locked(&smap->map, old_sdata->data,
> - value, false);
> + copy_map_value_locked_user(&smap->map, old_sdata->data,
> + value, false, from_user);
> return old_sdata;
> }
> }
> @@ -584,7 +586,7 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> /* A lookup has just been done before and concluded a new selem is
> * needed. The chance of an unnecessary alloc is unlikely.
> */
> - alloc_selem = selem = bpf_selem_alloc(smap, owner, value, true, gfp_flags);
> + alloc_selem = selem = bpf_selem_alloc(smap, owner, value, true, gfp_flags, from_user);
> if (!alloc_selem)
> return ERR_PTR(-ENOMEM);
>
> @@ -607,8 +609,8 @@ bpf_local_storage_update(void *owner, struct bpf_local_storage_map *smap,
> goto unlock;
>
> if (old_sdata && (map_flags & BPF_F_LOCK)) {
> - copy_map_value_locked(&smap->map, old_sdata->data, value,
> - false);
> + copy_map_value_locked_user(&smap->map, old_sdata->data, value,
> + false, from_user);
> selem = SELEM(old_sdata);
> goto unlock;
> }
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index d02ae323996b..4aef86209fdd 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -372,8 +372,8 @@ const struct bpf_func_proto bpf_spin_unlock_proto = {
> .arg1_btf_id = BPF_PTR_POISON,
> };
>
> -void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
> - bool lock_src)
> +void copy_map_value_locked_user(struct bpf_map *map, void *dst, void *src,
> + bool lock_src, bool from_user)
> {
> struct bpf_spin_lock *lock;
>
> @@ -383,11 +383,17 @@ void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
> lock = dst + map->record->spin_lock_off;
> preempt_disable();
> __bpf_spin_lock_irqsave(lock);
> - copy_map_value(map, dst, src);
> + copy_map_value_user(map, dst, src, from_user);
> __bpf_spin_unlock_irqrestore(lock);
> preempt_enable();
> }
>
> +void copy_map_value_locked(struct bpf_map *map, void *dst, void *src,
> + bool lock_src)
> +{
> + copy_map_value_locked_user(map, dst, src, lock_src, false);
> +}
> +
> BPF_CALL_0(bpf_jiffies64)
> {
> return get_jiffies_64();
> diff --git a/kernel/bpf/local_storage.c b/kernel/bpf/local_storage.c
> index 3969eb0382af..62a12fa8ce9e 100644
> --- a/kernel/bpf/local_storage.c
> +++ b/kernel/bpf/local_storage.c
> @@ -147,7 +147,7 @@ static long cgroup_storage_update_elem(struct bpf_map *map, void *key,
> struct bpf_cgroup_storage *storage;
> struct bpf_storage_buffer *new;
>
> - if (unlikely(flags & ~(BPF_F_LOCK | BPF_EXIST)))
> + if (unlikely(flags & ~BPF_F_LOCK))
> return -EINVAL;
>
> if (unlikely((flags & BPF_F_LOCK) &&
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 90a25307480e..eaa2a9d13265 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -155,8 +155,134 @@ static void maybe_wait_bpf_programs(struct bpf_map *map)
> synchronize_rcu();
> }
>
> -static int bpf_map_update_value(struct bpf_map *map, struct file *map_file,
> - void *key, void *value, __u64 flags)
> +static void *trans_addr_pages(struct page **pages, int npages)
> +{
> + if (npages == 1)
> + return page_address(pages[0]);
> + /* For multiple pages, we need to use vmap() to get a contiguous
> + * virtual address range.
> + */
> + return vmap(pages, npages, VM_MAP, PAGE_KERNEL);
> +}

Don't quite see a need for trans_addr_pages() helper when it's used
once.

> +
> +#define KPTR_USER_MAX_PAGES 16
> +
> +static int bpf_obj_trans_pin_uaddr(struct btf_field *field, void **addr)
> +{
> + const struct btf_type *t;
> + struct page *pages[KPTR_USER_MAX_PAGES];
> + void *ptr, *kern_addr;
> + u32 type_id, tsz;
> + int r, npages;
> +
> + ptr = *addr;
> + type_id = field->kptr.btf_id;
> + t = btf_type_id_size(field->kptr.btf, &type_id, &tsz);
> + if (!t)
> + return -EINVAL;
> + if (tsz == 0) {
> + *addr = NULL;
> + return 0;
> + }
> +
> + npages = (((intptr_t)ptr + tsz + ~PAGE_MASK) -
> + ((intptr_t)ptr & PAGE_MASK)) >> PAGE_SHIFT;
> + if (npages > KPTR_USER_MAX_PAGES)
> + return -E2BIG;
> + r = pin_user_pages_fast((intptr_t)ptr & PAGE_MASK, npages, 0, pages);

No need to apply the mask on ptr. See pin_user_pages_fast() internals.

It probably should be FOLL_WRITE | FOLL_LONGTERM instead of 0.

> + if (r != npages)
> + return -EINVAL;
> + kern_addr = trans_addr_pages(pages, npages);
> + if (!kern_addr)
> + return -ENOMEM;
> + *addr = kern_addr + ((intptr_t)ptr & ~PAGE_MASK);
> + return 0;
> +}
> +
> +void bpf_obj_unpin_uaddr(const struct btf_field *field, void *addr)
> +{
> + struct page *pages[KPTR_USER_MAX_PAGES];
> + int npages, i;
> + u32 size, type_id;
> + void *ptr;
> +
> + type_id = field->kptr.btf_id;
> + btf_type_id_size(field->kptr.btf, &type_id, &size);
> + if (size == 0)
> + return;
> +
> + ptr = (void *)((intptr_t)addr & PAGE_MASK);
> + npages = (((intptr_t)addr + size + ~PAGE_MASK) - (intptr_t)ptr) >> PAGE_SHIFT;
> + for (i = 0; i < npages; i++) {
> + pages[i] = virt_to_page(ptr);
> + ptr += PAGE_SIZE;
> + }
> + if (npages > 1)
> + /* Paired with vmap() in trans_addr_pages() */
> + vunmap((void *)((intptr_t)addr & PAGE_MASK));
> + unpin_user_pages(pages, npages);
> +}
> +
> +static int bpf_obj_trans_pin_uaddrs(struct btf_record *rec, void *src, u32 size)
> +{
> + u32 next_off;
> + int i, err;
> +
> + if (IS_ERR_OR_NULL(rec))
> + return 0;
> +
> + if (!btf_record_has_field(rec, BPF_KPTR_USER))
> + return 0;

imo kptr_user doesn't quite fit as a name.
'kptr' means 'kernel pointer'. Here it's user addr.
Maybe just "uptr" ?

> +
> + for (i = 0; i < rec->cnt; i++) {
> + if (rec->fields[i].type != BPF_KPTR_USER)
> + continue;
> +
> + next_off = rec->fields[i].offset;
> + if (next_off + sizeof(void *) > size)
> + return -EINVAL;
> + err = bpf_obj_trans_pin_uaddr(&rec->fields[i], src + next_off);
> + if (!err)
> + continue;
> +
> + /* Rollback */
> + for (i--; i >= 0; i--) {
> + if (rec->fields[i].type != BPF_KPTR_USER)
> + continue;
> + next_off = rec->fields[i].offset;
> + bpf_obj_unpin_uaddr(&rec->fields[i], *(void **)(src + next_off));
> + *(void **)(src + next_off) = NULL;
> + }
> +
> + return err;
> + }
> +
> + return 0;
> +}
> +
> +static void bpf_obj_unpin_uaddrs(struct btf_record *rec, void *src)
> +{
> + u32 next_off;
> + int i;
> +
> + if (IS_ERR_OR_NULL(rec))
> + return;
> +
> + if (!btf_record_has_field(rec, BPF_KPTR_USER))
> + return;
> +
> + for (i = 0; i < rec->cnt; i++) {
> + if (rec->fields[i].type != BPF_KPTR_USER)
> + continue;
> +
> + next_off = rec->fields[i].offset;
> + bpf_obj_unpin_uaddr(&rec->fields[i], *(void **)(src + next_off));
> + *(void **)(src + next_off) = NULL;

This part is pretty much the same as the undo part in
bpf_obj_trans_pin_uaddrs() and the common helper is warranted.

> + }
> +}
> +
> +static int bpf_map_update_value_inner(struct bpf_map *map, struct file *map_file,
> + void *key, void *value, __u64 flags)
> {
> int err;
>
> @@ -208,6 +334,29 @@ static int bpf_map_update_value(struct bpf_map *map, struct file *map_file,
> return err;
> }
>
> +static int bpf_map_update_value(struct bpf_map *map, struct file *map_file,
> + void *key, void *value, __u64 flags)
> +{
> + int err;
> +
> + if (flags & BPF_FROM_USER) {

there shouldn't be a need for this extra flag.
map->record has the info whether uptr is present or not.

> + /* Pin user memory can lead to context switch, so we need
> + * to do it before potential RCU lock.
> + */
> + err = bpf_obj_trans_pin_uaddrs(map->record, value,
> + bpf_map_value_size(map));
> + if (err)
> + return err;
> + }
> +
> + err = bpf_map_update_value_inner(map, map_file, key, value, flags);
> +
> + if (err && (flags & BPF_FROM_USER))
> + bpf_obj_unpin_uaddrs(map->record, value);
> +
> + return err;
> +}
> +
> static int bpf_map_copy_value(struct bpf_map *map, void *key, void *value,
> __u64 flags)
> {
> @@ -714,6 +863,11 @@ void bpf_obj_free_fields(const struct btf_record *rec, void *obj)
> field->kptr.dtor(xchgd_field);
> }
> break;
> + case BPF_KPTR_USER:
> + if (virt_addr_valid(*(void **)field_ptr))
> + bpf_obj_unpin_uaddr(field, *(void **)field_ptr);
> + *(void **)field_ptr = NULL;
> + break;
> case BPF_LIST_HEAD:
> if (WARN_ON_ONCE(rec->spin_lock_off < 0))
> continue;
> @@ -1155,6 +1309,12 @@ static int map_check_btf(struct bpf_map *map, struct bpf_token *token,
> goto free_map_tab;
> }
> break;
> + case BPF_KPTR_USER:
> + if (map->map_type != BPF_MAP_TYPE_TASK_STORAGE) {
> + ret = -EOPNOTSUPP;
> + goto free_map_tab;
> + }
> + break;
> case BPF_LIST_HEAD:
> case BPF_RB_ROOT:
> if (map->map_type != BPF_MAP_TYPE_HASH &&
> @@ -1618,11 +1778,15 @@ static int map_update_elem(union bpf_attr *attr, bpfptr_t uattr)
> struct bpf_map *map;
> void *key, *value;
> u32 value_size;
> + u64 extra_flags = 0;
> struct fd f;
> int err;
>
> if (CHECK_ATTR(BPF_MAP_UPDATE_ELEM))
> return -EINVAL;
> + /* Prevent userspace from setting any internal flags */
> + if (attr->flags & ~(BIT(BPF_MAP_UPDATE_FLAG_BITS) - 1))
> + return -EINVAL;
>
> f = fdget(ufd);
> map = __bpf_map_get(f);
> @@ -1653,7 +1817,9 @@ static int map_update_elem(union bpf_attr *attr, bpfptr_t uattr)
> goto free_key;
> }
>
> - err = bpf_map_update_value(map, f.file, key, value, attr->flags);
> + if (map->map_type == BPF_MAP_TYPE_TASK_STORAGE)
> + extra_flags |= BPF_FROM_USER;
> + err = bpf_map_update_value(map, f.file, key, value, attr->flags | extra_flags);
> if (!err)
> maybe_wait_bpf_programs(map);
>
> @@ -1852,6 +2018,7 @@ int generic_map_update_batch(struct bpf_map *map, struct file *map_file,
> void __user *keys = u64_to_user_ptr(attr->batch.keys);
> u32 value_size, cp, max_count;
> void *key, *value;
> + u64 extra_flags = 0;
> int err = 0;
>
> if (attr->batch.elem_flags & ~BPF_F_LOCK)
> @@ -1881,6 +2048,8 @@ int generic_map_update_batch(struct bpf_map *map, struct file *map_file,
> return -ENOMEM;
> }
>
> + if (map->map_type == BPF_MAP_TYPE_TASK_STORAGE)
> + extra_flags |= BPF_FROM_USER;
> for (cp = 0; cp < max_count; cp++) {
> err = -EFAULT;
> if (copy_from_user(key, keys + cp * map->key_size,
> @@ -1889,7 +2058,7 @@ int generic_map_update_batch(struct bpf_map *map, struct file *map_file,
> break;
>
> err = bpf_map_update_value(map, map_file, key, value,
> - attr->batch.elem_flags);
> + attr->batch.elem_flags | extra_flags);
>
> if (err)
> break;
> diff --git a/net/core/bpf_sk_storage.c b/net/core/bpf_sk_storage.c
> index bc01b3aa6b0f..db5281384e6a 100644
> --- a/net/core/bpf_sk_storage.c
> +++ b/net/core/bpf_sk_storage.c
> @@ -137,7 +137,7 @@ bpf_sk_storage_clone_elem(struct sock *newsk,
> {
> struct bpf_local_storage_elem *copy_selem;
>
> - copy_selem = bpf_selem_alloc(smap, newsk, NULL, true, GFP_ATOMIC);
> + copy_selem = bpf_selem_alloc(smap, newsk, NULL, true, GFP_ATOMIC, false);
> if (!copy_selem)
> return NULL;
>
> --
> 2.34.1
>