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Message-ID: <8e62ae3f8cf94c798fc1b7ffd69cbdc4@huawei.com>
Date: Wed, 28 Apr 2021 15:35:28 +0000
From: Roberto Sassu <roberto.sassu@...wei.com>
To: "zohar@...ux.ibm.com" <zohar@...ux.ibm.com>,
"mjg59@...gle.com" <mjg59@...gle.com>
CC: "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
"linux-security-module@...r.kernel.org"
<linux-security-module@...r.kernel.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Casey Schaufler <casey@...aufler-ca.com>
Subject: RE: [PATCH v4 04/11] ima: Move ima_reset_appraise_flags() call to
post hooks
> From: Roberto Sassu
> Sent: Friday, March 5, 2021 4:19 PM
> ima_inode_setxattr() and ima_inode_removexattr() hooks are called before
> an
> operation is performed. Thus, ima_reset_appraise_flags() should not be
> called there, as flags might be unnecessarily reset if the operation is
> denied.
>
> This patch introduces the post hooks ima_inode_post_setxattr() and
> ima_inode_post_removexattr(), and adds the call to
> ima_reset_appraise_flags() in the new functions.
>
> Cc: Casey Schaufler <casey@...aufler-ca.com>
> Signed-off-by: Roberto Sassu <roberto.sassu@...wei.com>
> ---
> fs/xattr.c | 2 ++
> include/linux/ima.h | 18 ++++++++++++++++++
> security/integrity/ima/ima_appraise.c | 25 ++++++++++++++++++++++---
> security/security.c | 1 +
> 4 files changed, 43 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xattr.c b/fs/xattr.c
> index b3444e06cded..81847f132d26 100644
> --- a/fs/xattr.c
> +++ b/fs/xattr.c
> @@ -16,6 +16,7 @@
> #include <linux/namei.h>
> #include <linux/security.h>
> #include <linux/evm.h>
> +#include <linux/ima.h>
> #include <linux/syscalls.h>
> #include <linux/export.h>
> #include <linux/fsnotify.h>
> @@ -502,6 +503,7 @@ __vfs_removexattr_locked(struct user_namespace
> *mnt_userns,
>
> if (!error) {
> fsnotify_xattr(dentry);
> + ima_inode_post_removexattr(dentry, name);
> evm_inode_post_removexattr(dentry, name);
> }
>
> diff --git a/include/linux/ima.h b/include/linux/ima.h
> index 61d5723ec303..5e059da43857 100644
> --- a/include/linux/ima.h
> +++ b/include/linux/ima.h
> @@ -171,7 +171,13 @@ extern void ima_inode_post_setattr(struct
> user_namespace *mnt_userns,
> struct dentry *dentry);
> extern int ima_inode_setxattr(struct dentry *dentry, const char *xattr_name,
> const void *xattr_value, size_t xattr_value_len);
> +extern void ima_inode_post_setxattr(struct dentry *dentry,
> + const char *xattr_name,
> + const void *xattr_value,
> + size_t xattr_value_len);
> extern int ima_inode_removexattr(struct dentry *dentry, const char
> *xattr_name);
> +extern void ima_inode_post_removexattr(struct dentry *dentry,
> + const char *xattr_name);
> #else
> static inline bool is_ima_appraise_enabled(void)
> {
> @@ -192,11 +198,23 @@ static inline int ima_inode_setxattr(struct dentry
> *dentry,
> return 0;
> }
>
> +static inline void ima_inode_post_setxattr(struct dentry *dentry,
> + const char *xattr_name,
> + const void *xattr_value,
> + size_t xattr_value_len)
> +{
> +}
> +
> static inline int ima_inode_removexattr(struct dentry *dentry,
> const char *xattr_name)
> {
> return 0;
> }
> +
> +static inline void ima_inode_post_removexattr(struct dentry *dentry,
> + const char *xattr_name)
> +{
> +}
> #endif /* CONFIG_IMA_APPRAISE */
>
> #if defined(CONFIG_IMA_APPRAISE) &&
> defined(CONFIG_INTEGRITY_TRUSTED_KEYRING)
> diff --git a/security/integrity/ima/ima_appraise.c
> b/security/integrity/ima/ima_appraise.c
> index 565e33ff19d0..1f029e4c8d7f 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -577,21 +577,40 @@ int ima_inode_setxattr(struct dentry *dentry, const
> char *xattr_name,
> if (result == 1) {
> if (!xattr_value_len || (xvalue->type >= IMA_XATTR_LAST))
> return -EINVAL;
> - ima_reset_appraise_flags(d_backing_inode(dentry),
> - xvalue->type == EVM_IMA_XATTR_DIGSIG);
> result = 0;
> }
> return result;
> }
>
> +void ima_inode_post_setxattr(struct dentry *dentry, const char
> *xattr_name,
> + const void *xattr_value, size_t xattr_value_len)
> +{
> + const struct evm_ima_xattr_data *xvalue = xattr_value;
> + int result;
> +
> + result = ima_protect_xattr(dentry, xattr_name, xattr_value,
> + xattr_value_len);
> + if (result == 1)
> + ima_reset_appraise_flags(d_backing_inode(dentry),
I found an issue in this patch.
Moving ima_reset_appraise_flags() to the post hook causes this
function to be executed also when __vfs_setxattr_noperm() is
called.
The problem is that at the end of a write IMA calls
ima_collect_measurement() to recalculate the file digest and
update security.ima. ima_collect_measurement() sets
IMA_COLLECTED.
However, after that __vfs_setxattr_noperm() causes
IMA_COLLECTED to be reset, and to unnecessarily recalculate
the file digest. This wouldn't happen if ima_reset_appraise_flags()
is in the pre hook.
I solved by replacing:
iint->flags &= ~IMA_DONE_MASK;
with:
iint->flags &= ~(IMA_DONE_MASK & ~IMA_COLLECTED);
just when the IMA_CHANGE_XATTR bit is set. It should
not be a problem since setting an xattr does not influence
the file content.
Mimi, what do you think?
Thanks
Roberto
HUAWEI TECHNOLOGIES Duesseldorf GmbH, HRB 56063
Managing Director: Li Peng, Li Jian, Shi Yanli
> + xvalue->type == EVM_IMA_XATTR_DIGSIG);
> +}
> +
> int ima_inode_removexattr(struct dentry *dentry, const char *xattr_name)
> {
> int result;
>
> result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
> if (result == 1) {
> - ima_reset_appraise_flags(d_backing_inode(dentry), 0);
> result = 0;
> }
> return result;
> }
> +
> +void ima_inode_post_removexattr(struct dentry *dentry, const char
> *xattr_name)
> +{
> + int result;
> +
> + result = ima_protect_xattr(dentry, xattr_name, NULL, 0);
> + if (result == 1)
> + ima_reset_appraise_flags(d_backing_inode(dentry), 0);
> +}
> diff --git a/security/security.c b/security/security.c
> index 5ac96b16f8fa..efb1f874dc41 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1319,6 +1319,7 @@ void security_inode_post_setxattr(struct dentry
> *dentry, const char *name,
> if (unlikely(IS_PRIVATE(d_backing_inode(dentry))))
> return;
> call_void_hook(inode_post_setxattr, dentry, name, value, size, flags);
> + ima_inode_post_setxattr(dentry, name, value, size);
> evm_inode_post_setxattr(dentry, name, value, size);
> }
>
> --
> 2.26.2
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