* [PATCH v2 2/2] smack: fix bug: setting task label silently ignores input garbage
2025-03-15 1:57 [PATCH v2 0/2] smack: fix two bugs in setting task label Konstantin Andreev
2025-03-15 1:57 ` [PATCH v2 1/2] smack: fix bug: unprivileged task can create labels Konstantin Andreev
@ 2025-03-15 1:57 ` Konstantin Andreev
2025-06-16 1:52 ` [PATCH v2 0/2] smack: fix two bugs in setting task label Konstantin Andreev
2 siblings, 0 replies; 8+ messages in thread
From: Konstantin Andreev @ 2025-03-15 1:57 UTC (permalink / raw)
To: Casey Schaufler; +Cc: linux-security-module
This command:
# echo foo/bar >/proc/$$/attr/smack/current
gives the task a label 'foo' w/o indication
that label does not match input.
Setting the label with lsm_set_self_attr() syscall
behaves identically.
This occures because:
1) smk_parse_smack() is used to convert input to a label
2) smk_parse_smack() takes only that part from the
beginning of the input that looks like a label.
3) `/' is prohibited in labels, so only "foo" is taken.
(2) is by design, because smk_parse_smack() is used
for parsing strings which are more than just a label.
Silent failure is not a good thing, and there are two
indicators that this was not done intentionally:
(size >= SMK_LONGLABEL) ~> invalid
clause at the beginning of the do_setattr() and the
"Returns the length of the smack label" claim
in the do_setattr() description.
So I fixed this by adding one tiny check:
the taken label length == input length.
Since input length is now strictly controlled,
I changed the two ways of setting label
smack_setselfattr(): lsm_set_self_attr() syscall
smack_setprocattr(): > /proc/.../current
to accommodate the divergence in
what they understand by "input length":
smack_setselfattr counts mandatory \0 into input length,
smack_setprocattr does not.
smack_setprocattr allows various trailers after label
Related changes:
* fixed description for smk_parse_smack
* allow unprivileged tasks validate label syntax.
* extract smk_parse_label_len() from smk_parse_smack()
so parsing may be done w/o string allocation.
* extract smk_import_valid_label() from smk_import_entry()
to avoid repeated parsing.
* smk_parse_smack(): scan null-terminated strings
for no more than SMK_LONGLABEL(256) characters
* smack_setselfattr(): require struct lsm_ctx . flags == 0
to reserve them for future.
Fixes: e114e473771c ("Smack: Simplified Mandatory Access Control Kernel")
Signed-off-by: Konstantin Andreev <andreev@swemel.ru>
---
v2: 1) accept two more trailers after label
in /proc/PID/attr/smack/current input: \0 and \n\0
2) added documentation
Documentation/admin-guide/LSM/Smack.rst | 11 ++-
security/smack/smack.h | 3 +
security/smack/smack_access.c | 93 ++++++++++++++-----
security/smack/smack_lsm.c | 117 +++++++++++++++---------
4 files changed, 157 insertions(+), 67 deletions(-)
diff --git a/Documentation/admin-guide/LSM/Smack.rst b/Documentation/admin-guide/LSM/Smack.rst
index 6d44f4fdbf59..2b89b5dfddea 100644
--- a/Documentation/admin-guide/LSM/Smack.rst
+++ b/Documentation/admin-guide/LSM/Smack.rst
@@ -601,10 +601,15 @@ specification.
Task Attribute
~~~~~~~~~~~~~~
-The Smack label of a process can be read from /proc/<pid>/attr/current. A
-process can read its own Smack label from /proc/self/attr/current. A
+The Smack label of a process can be read from ``/proc/<pid>/attr/current``. A
+process can read its own Smack label from ``/proc/self/attr/current``. A
privileged process can change its own Smack label by writing to
-/proc/self/attr/current but not the label of another process.
+``/proc/self/attr/current`` but not the label of another process.
+
+Format of writing is : only the label or the label followed by one of the
+3 trailers: ``\n`` (by common agreement for ``/proc/...`` interfaces),
+``\0`` (because some applications incorrectly include it),
+``\n\0`` (because we think some applications may incorrectly include it).
File Attribute
~~~~~~~~~~~~~~
diff --git a/security/smack/smack.h b/security/smack/smack.h
index bf6a6ed3946c..759343a6bbae 100644
--- a/security/smack/smack.h
+++ b/security/smack/smack.h
@@ -286,9 +286,12 @@ int smk_tskacc(struct task_smack *, struct smack_known *,
int smk_curacc(struct smack_known *, u32, struct smk_audit_info *);
int smack_str_from_perm(char *string, int access);
struct smack_known *smack_from_secid(const u32);
+int smk_parse_label_len(const char *string, int len);
char *smk_parse_smack(const char *string, int len);
int smk_netlbl_mls(int, char *, struct netlbl_lsm_secattr *, int);
struct smack_known *smk_import_entry(const char *, int);
+struct smack_known *smk_import_valid_label(const char *label, int label_len,
+ gfp_t gfp);
void smk_insert_entry(struct smack_known *skp);
struct smack_known *smk_find_entry(const char *);
bool smack_privileged(int cap);
diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
index 2e4a0cb22782..a289cb6672bd 100644
--- a/security/smack/smack_access.c
+++ b/security/smack/smack_access.c
@@ -443,19 +443,19 @@ struct smack_known *smk_find_entry(const char *string)
}
/**
- * smk_parse_smack - parse smack label from a text string
- * @string: a text string that might contain a Smack label
- * @len: the maximum size, or zero if it is NULL terminated.
+ * smk_parse_label_len - calculate the length of the starting segment
+ * in the string that constitutes a valid smack label
+ * @string: a text string that might contain a Smack label at the beginning
+ * @len: the maximum size to look into, may be zero if string is null-terminated
*
- * Returns a pointer to the clean label or an error code.
+ * Returns the length of the segment (0 < L < SMK_LONGLABEL) or an error code.
*/
-char *smk_parse_smack(const char *string, int len)
+int smk_parse_label_len(const char *string, int len)
{
- char *smack;
int i;
- if (len <= 0)
- len = strlen(string) + 1;
+ if (len <= 0 || len > SMK_LONGLABEL)
+ len = SMK_LONGLABEL;
/*
* Reserve a leading '-' as an indicator that
@@ -463,7 +463,7 @@ char *smk_parse_smack(const char *string, int len)
* including /smack/cipso and /smack/cipso2
*/
if (string[0] == '-')
- return ERR_PTR(-EINVAL);
+ return -EINVAL;
for (i = 0; i < len; i++)
if (string[i] > '~' || string[i] <= ' ' || string[i] == '/' ||
@@ -471,6 +471,25 @@ char *smk_parse_smack(const char *string, int len)
break;
if (i == 0 || i >= SMK_LONGLABEL)
+ return -EINVAL;
+
+ return i;
+}
+
+/**
+ * smk_parse_smack - copy the starting segment in the string
+ * that constitutes a valid smack label
+ * @string: a text string that might contain a Smack label at the beginning
+ * @len: the maximum size to look into, may be zero if string is null-terminated
+ *
+ * Returns a pointer to the copy of the label or an error code.
+ */
+char *smk_parse_smack(const char *string, int len)
+{
+ char *smack;
+ int i = smk_parse_label_len(string, len);
+
+ if (i < 0)
return ERR_PTR(-EINVAL);
smack = kstrndup(string, i, GFP_NOFS);
@@ -554,31 +573,25 @@ int smack_populate_secattr(struct smack_known *skp)
}
/**
- * smk_import_entry - import a label, return the list entry
- * @string: a text string that might be a Smack label
- * @len: the maximum size, or zero if it is NULL terminated.
+ * smk_import_valid_allocated_label - import a label, return the list entry
+ * @smack: a text string that is a valid Smack label and may be kfree()ed.
+ * It is consumed: either becomes a part of the entry or kfree'ed.
*
- * Returns a pointer to the entry in the label list that
- * matches the passed string, adding it if necessary,
- * or an error code.
+ * Returns: see description of smk_import_entry()
*/
-struct smack_known *smk_import_entry(const char *string, int len)
+static struct smack_known *
+smk_import_allocated_label(char *smack, gfp_t gfp)
{
struct smack_known *skp;
- char *smack;
int rc;
- smack = smk_parse_smack(string, len);
- if (IS_ERR(smack))
- return ERR_CAST(smack);
-
mutex_lock(&smack_known_lock);
skp = smk_find_entry(smack);
if (skp != NULL)
goto freeout;
- skp = kzalloc(sizeof(*skp), GFP_NOFS);
+ skp = kzalloc(sizeof(*skp), gfp);
if (skp == NULL) {
skp = ERR_PTR(-ENOMEM);
goto freeout;
@@ -608,6 +621,42 @@ struct smack_known *smk_import_entry(const char *string, int len)
return skp;
}
+/**
+ * smk_import_entry - import a label, return the list entry
+ * @string: a text string that might contain a Smack label at the beginning
+ * @len: the maximum size to look into, may be zero if string is null-terminated
+ *
+ * Returns a pointer to the entry in the label list that
+ * matches the passed string, adding it if necessary,
+ * or an error code.
+ */
+struct smack_known *smk_import_entry(const char *string, int len)
+{
+ char *smack = smk_parse_smack(string, len);
+
+ if (IS_ERR(smack))
+ return ERR_CAST(smack);
+
+ return smk_import_allocated_label(smack, GFP_NOFS);
+}
+
+/**
+ * smk_import_valid_label - import a label, return the list entry
+ * @label a text string that is a valid Smack label, not null-terminated
+ *
+ * Returns: see description of smk_import_entry()
+ */
+struct smack_known *
+smk_import_valid_label(const char *label, int label_len, gfp_t gfp)
+{
+ char *smack = kstrndup(label, label_len, gfp);
+
+ if (!smack)
+ return ERR_PTR(-ENOMEM);
+
+ return smk_import_allocated_label(smack, gfp);
+}
+
/**
* smack_from_secid - find the Smack label associated with a secid
* @secid: an integer that might be associated with a Smack label
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 80d68d2c228c..4a463da8f37e 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -3664,7 +3664,7 @@ static void smack_d_instantiate(struct dentry *opt_dentry, struct inode *inode)
* @attr: which attribute to fetch
* @ctx: buffer to receive the result
* @size: available size in, actual size out
- * @flags: unused
+ * @flags: reserved, currently zero
*
* Fill the passed user space @ctx with the details of the requested
* attribute.
@@ -3725,57 +3725,52 @@ static int smack_getprocattr(struct task_struct *p, const char *name, char **val
* Sets the Smack value of the task. Only setting self
* is permitted and only with privilege
*
- * Returns the length of the smack label or an error code
+ * Returns zero on success or an error code
*/
-static int do_setattr(u64 attr, void *value, size_t size)
+static int do_setattr(unsigned int attr, void *value, size_t size)
{
struct task_smack *tsp = smack_cred(current_cred());
struct cred *new;
struct smack_known *skp;
- char *labelstr;
- int rc = 0;
-
- if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
- return -EPERM;
+ int label_len;
+ /*
+ * let unprivileged user validate input, check permissions later
+ */
if (value == NULL || size == 0 || size >= SMK_LONGLABEL)
return -EINVAL;
- if (attr != LSM_ATTR_CURRENT)
- return -EOPNOTSUPP;
-
- labelstr = smk_parse_smack(value, size);
- if (IS_ERR(labelstr))
- return PTR_ERR(labelstr);
+ label_len = smk_parse_label_len(value, size);
+ if (label_len < 0 || label_len != size)
+ return -EINVAL;
/*
* No process is ever allowed the web ("@") label
* and the star ("*") label.
*/
- if (labelstr[1] == '\0' /* '@', '*' */) {
- const char c = labelstr[0];
+ if (label_len == 1 /* '@', '*' */) {
+ const char c = *(const char *)value;
if (c == *smack_known_web.smk_known ||
- c == *smack_known_star.smk_known) {
- rc = -EPERM;
- goto free_labelstr;
- }
+ c == *smack_known_star.smk_known)
+ return -EPERM;
}
if (!smack_privileged(CAP_MAC_ADMIN)) {
const struct smack_known_list_elem *sklep;
- list_for_each_entry(sklep, &tsp->smk_relabel, list)
- if (strcmp(sklep->smk_label->smk_known, labelstr) == 0)
- goto free_labelstr;
- rc = -EPERM;
+ list_for_each_entry(sklep, &tsp->smk_relabel, list) {
+ const char *cp = sklep->smk_label->smk_known;
+
+ if (strlen(cp) == label_len &&
+ strncmp(cp, value, label_len) == 0)
+ goto in_relabel;
+ }
+ return -EPERM;
+in_relabel:
+ ;
}
-free_labelstr:
- kfree(labelstr);
- if (rc)
- return -EPERM;
-
- skp = smk_import_entry(value, size);
+ skp = smk_import_valid_label(value, label_len, GFP_KERNEL);
if (IS_ERR(skp))
return PTR_ERR(skp);
@@ -3791,7 +3786,7 @@ static int do_setattr(u64 attr, void *value, size_t size)
smk_destroy_label_list(&tsp->smk_relabel);
commit_creds(new);
- return size;
+ return 0;
}
/**
@@ -3799,7 +3794,7 @@ static int do_setattr(u64 attr, void *value, size_t size)
* @attr: which attribute to set
* @ctx: buffer containing the data
* @size: size of @ctx
- * @flags: unused
+ * @flags: reserved, must be zero
*
* Fill the passed user space @ctx with the details of the requested
* attribute.
@@ -3809,12 +3804,26 @@ static int do_setattr(u64 attr, void *value, size_t size)
static int smack_setselfattr(unsigned int attr, struct lsm_ctx *ctx,
u32 size, u32 flags)
{
- int rc;
+ if (attr != LSM_ATTR_CURRENT)
+ return -EOPNOTSUPP;
- rc = do_setattr(attr, ctx->ctx, ctx->ctx_len);
- if (rc > 0)
- return 0;
- return rc;
+ if (ctx->flags)
+ return -EINVAL;
+ /*
+ * string must have \0 terminator, included in ctx->ctx
+ * (see description of struct lsm_ctx)
+ */
+ if (ctx->ctx_len == 0)
+ return -EINVAL;
+
+ if (ctx->ctx[ctx->ctx_len - 1] != '\0')
+ return -EINVAL;
+ /*
+ * other do_setattr() caller, smack_setprocattr(),
+ * does not count \0 into size, so
+ * decreasing length by 1 to accommodate the divergence.
+ */
+ return do_setattr(attr, ctx->ctx, ctx->ctx_len - 1);
}
/**
@@ -3826,15 +3835,39 @@ static int smack_setselfattr(unsigned int attr, struct lsm_ctx *ctx,
* Sets the Smack value of the task. Only setting self
* is permitted and only with privilege
*
- * Returns the length of the smack label or an error code
+ * Returns the size of the input value or an error code
*/
static int smack_setprocattr(const char *name, void *value, size_t size)
{
- int attr = lsm_name_to_attr(name);
+ size_t realsize = size;
+ unsigned int attr = lsm_name_to_attr(name);
- if (attr != LSM_ATTR_UNDEF)
- return do_setattr(attr, value, size);
- return -EINVAL;
+ switch (attr) {
+ case LSM_ATTR_UNDEF: return -EINVAL;
+ default: return -EOPNOTSUPP;
+ case LSM_ATTR_CURRENT:
+ ;
+ }
+
+ /*
+ * The value for the "current" attribute is the label
+ * followed by one of the 4 trailers: none, \0, \n, \n\0
+ *
+ * I.e. following inputs are accepted as 3-characters long label "foo":
+ *
+ * "foo" (3 characters)
+ * "foo\0" (4 characters)
+ * "foo\n" (4 characters)
+ * "foo\n\0" (5 characters)
+ */
+
+ if (realsize && (((const char *)value)[realsize - 1] == '\0'))
+ --realsize;
+
+ if (realsize && (((const char *)value)[realsize - 1] == '\n'))
+ --realsize;
+
+ return do_setattr(attr, value, realsize) ? : size;
}
/**
--
2.43.0
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