| Message ID | 20240725234706.655613-2-song@kernel.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Add kfuncs to support reading xattr from dentry | expand |
On Thu, Jul 25, 2024 at 04:47:05PM -0700, Song Liu wrote: > +__bpf_kfunc struct dentry *bpf_file_dentry(const struct file *file) > +{ > + /* file_dentry() does not hold reference to the dentry. We add a > + * dget() here so that we can add KF_ACQUIRE flag to > + * bpf_file_dentry(). > + */ > + return dget(file_dentry(file)); > +} > + > +__bpf_kfunc struct dentry *bpf_dget_parent(struct dentry *dentry) > +{ > + return dget_parent(dentry); > +} > + > +__bpf_kfunc void bpf_dput(struct dentry *dentry) > +{ > + return dput(dentry); > +} If you keep a file reference, why bother grabbing dentry one? If not, you have a very bad trouble if that opened file is the only thing that keeps the filesystem busy. It's almost certainly a wrong interface; please, explain what exactly are you trying to do here.
Hi Al, Thanks for your quick reply. > On Jul 25, 2024, at 10:34 PM, Al Viro <viro@zeniv.linux.org.uk> wrote: > > On Thu, Jul 25, 2024 at 04:47:05PM -0700, Song Liu wrote: > >> +__bpf_kfunc struct dentry *bpf_file_dentry(const struct file *file) >> +{ >> + /* file_dentry() does not hold reference to the dentry. We add a >> + * dget() here so that we can add KF_ACQUIRE flag to >> + * bpf_file_dentry(). >> + */ >> + return dget(file_dentry(file)); >> +} >> + >> +__bpf_kfunc struct dentry *bpf_dget_parent(struct dentry *dentry) >> +{ >> + return dget_parent(dentry); >> +} >> + >> +__bpf_kfunc void bpf_dput(struct dentry *dentry) >> +{ >> + return dput(dentry); >> +} > > If you keep a file reference, why bother grabbing dentry one? > If not, you have a very bad trouble if that opened file is the only > thing that keeps the filesystem busy. Yes, we keep a file reference for the duration of the BPF program. Therefore, it is technically not necessary to grab a dentry one. However, we grab a dentry reference to make the dentry pointer returned by bpf_file_dentry() a trusted pointer from BPF verifier's POV, so that these kfuncs are more robust. The following explanation is a bit long. Please let me know if it turns out confusing. ==== What is trusted pointer? ==== Trusted point is the mechanism to make sure bpf kfuncs are called with valid pointer. The BPF verifier requires certain BPF kfuncs (helpers) are called with trusted pointers. A pointer is trusted if one of the following two is true: 1. The pointer is passed directly by the tracepoint/kprobe, i.e., no pointer walking, no non-zero offset. For example, int bpf_security_file_open(struct file *file) /* file is trusted */ { /* mapping is not trusted */ struct address_space *mapping = file->f_mapping; /* file2 is not trusted */ struct file *file2 = file + 1; } 2. The pointer is returned by a kfunc with KF_ACQUIRE. This pointer has to be released by a kfunc with KF_RELEASE. KF_ACQUIRE and KF_RELEASE kfuncs are like any _get() _put() pairs. ==== bpf_dget_parent and bpf_dput ==== In this case, bpf_dget_parent() is a KF_ACQUIRE kfunc and bpf_dput() is a KF_RELEASE function. They are just like regular _get() _put() functions. The BPF verifier makes sure pointers acquired by bpf_dget_parent() is always released by bpf_dput() before the BPF program returns. For example, in the following BPF program: xxxx(struct dentry *d) { struct dentry *parent = bpf_dget_parent(d); /* main logic */ bpf_dput(parent); } If the bpf_dput() call is missing, the verifier will not allow the program to load. ==== More on kfunc safety ==== Trusted point makes kfunc calls safe. In this case, we want bpf_get_dentry_xattr() to only take trusted dentry pointer. For example, in the security_inode_listxattr LSM hook: bpf_security_inode_listxattr(struct dentry *dentry) { /* This is allowed, dentry is an input and thus * is trusted */ bpf_get_dentry_xattr(dentry); /* This is not allowed, as dentry->d_parent is * not trusted */ bpf_get_dentry_xattr(dentry->d_parent); /* This is allowed, as bpf_dget_parent() holds * a reference to d_parent, and returns a trusted * pointer */ struct dentry *parent = bpf_dget_parent(dentry); /* The following is needed, as we need the release * parent pointer. If this line is missing, this * program cannot pass BPF verifier. */ bpf_dput(parent); } ==== bpf_file_dentry ==== In this use case, we want to get from file pointer, such as LSM hook security_file_open() to the dentry and thus walk the directory tree. However, security_file_open() does not pass in a dentry pointer, and file->f_path.dentry is not a trusted pointer. There are two ways to get a trusted dentry pointer from a file pointer: 1. As what we do here, use bpf_file_dentry() to hold a reference on file->f_path.dentry and return a trusted pointer. 2. Give the verifier special knowledge that if file pointer is trusted, file->f_path.dentry is also trusted. This can be achieve with the following macros: BTF_TYPE_SAFE_TRUSTED BTF_TYPE_SAFE_RCU BTF_TYPE_SAFE_RCU_OR_NULL. Using the second method here requires a little more work in the BPF verifier, as dentry is not a simple pointer in struct file, but f_path.dentry. Therefore, I chose current approach that bpf_file_dentry() holds a reference on dentry pointer, and the pointer has to be released with bpf_dput(). For more details about trusted pointers in kfuncs, please refer to Documentation/bpf/kfuncs.rst. Does this answer your question? Thanks, Song > It's almost certainly a wrong interface; please, explain what > exactly are you trying to do here.
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c index cd098846e251..b8e6eeabb773 100644 --- a/kernel/trace/bpf_trace.c +++ b/kernel/trace/bpf_trace.c @@ -1443,22 +1443,21 @@ late_initcall(bpf_key_sig_kfuncs_init); __bpf_kfunc_start_defs(); /** - * bpf_get_file_xattr - get xattr of a file - * @file: file to get xattr from + * bpf_get_dentry_xattr - get xattr of a dentry + * @dentry: dentry to get xattr from * @name__str: name of the xattr * @value_p: output buffer of the xattr value * - * Get xattr *name__str* of *file* and store the output in *value_ptr*. + * Get xattr *name__str* of *dentry* and store the output in *value_ptr*. * * For security reasons, only *name__str* with prefix "user." is allowed. * * Return: 0 on success, a negative value on error. */ -__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str, - struct bpf_dynptr *value_p) +__bpf_kfunc int bpf_get_dentry_xattr(struct dentry *dentry, const char *name__str, + struct bpf_dynptr *value_p) { struct bpf_dynptr_kern *value_ptr = (struct bpf_dynptr_kern *)value_p; - struct dentry *dentry; u32 value_len; void *value; int ret; @@ -1471,20 +1470,63 @@ __bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str, if (!value) return -EINVAL; - dentry = file_dentry(file); ret = inode_permission(&nop_mnt_idmap, dentry->d_inode, MAY_READ); if (ret) return ret; return __vfs_getxattr(dentry, dentry->d_inode, name__str, value, value_len); } +/** + * bpf_get_file_xattr - get xattr of a file + * @file: file to get xattr from + * @name__str: name of the xattr + * @value_p: output buffer of the xattr value + * + * Get xattr *name__str* of *file* and store the output in *value_ptr*. + * + * For security reasons, only *name__str* with prefix "user." is allowed. + * + * Return: 0 on success, a negative value on error. + */ +__bpf_kfunc int bpf_get_file_xattr(struct file *file, const char *name__str, + struct bpf_dynptr *value_p) +{ + struct dentry *dentry; + + dentry = file_dentry(file); + return bpf_get_dentry_xattr(dentry, name__str, value_p); +} + +__bpf_kfunc struct dentry *bpf_file_dentry(const struct file *file) +{ + /* file_dentry() does not hold reference to the dentry. We add a + * dget() here so that we can add KF_ACQUIRE flag to + * bpf_file_dentry(). + */ + return dget(file_dentry(file)); +} + +__bpf_kfunc struct dentry *bpf_dget_parent(struct dentry *dentry) +{ + return dget_parent(dentry); +} + +__bpf_kfunc void bpf_dput(struct dentry *dentry) +{ + return dput(dentry); +} + __bpf_kfunc_end_defs(); BTF_KFUNCS_START(fs_kfunc_set_ids) +BTF_ID_FLAGS(func, bpf_get_dentry_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS) BTF_ID_FLAGS(func, bpf_get_file_xattr, KF_SLEEPABLE | KF_TRUSTED_ARGS) +BTF_ID_FLAGS(func, bpf_file_dentry, KF_TRUSTED_ARGS | KF_ACQUIRE) +BTF_ID_FLAGS(func, bpf_dget_parent, KF_TRUSTED_ARGS | KF_ACQUIRE) +BTF_ID_FLAGS(func, bpf_dput, KF_RELEASE) BTF_KFUNCS_END(fs_kfunc_set_ids) -static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id) +static int fs_kfunc_filter(const struct bpf_prog *prog, u32 kfunc_id) { if (!btf_id_set8_contains(&fs_kfunc_set_ids, kfunc_id)) return 0; @@ -1496,7 +1538,7 @@ static int bpf_get_file_xattr_filter(const struct bpf_prog *prog, u32 kfunc_id) static const struct btf_kfunc_id_set bpf_fs_kfunc_set = { .owner = THIS_MODULE, .set = &fs_kfunc_set_ids, - .filter = bpf_get_file_xattr_filter, + .filter = fs_kfunc_filter, }; static int __init bpf_fs_kfuncs_init(void)
The primary use case here is to read xattr for directories from BPF programs. This feature enables tagging all the files in a directory with a xattr on the directory. More specifically, starting from LSM hook security_file_open(), we can read xattr of the file's parent directories. To have referenced access to dentry, a few more kfuncs are added: - bpf_file_dentry - bpf_dget_parent - bpf_dput Both bpf_file_dentry and bpf_dget_parent take a reference to the dentry (KF_ACQUIRE), which has to be released by bpf_dput (KF_RELEASE). This makes sure only trusted pointers (KF_TRUSTED_ARGS) can be used for the dentry. Note that, file_dentry() doesn't take reference to the dentry, but bpf_file_dentry() does. Signed-off-by: Song Liu <song@kernel.org> --- kernel/trace/bpf_trace.c | 60 ++++++++++++++++++++++++++++++++++------ 1 file changed, 51 insertions(+), 9 deletions(-)