On Thu, Sep 04, 2025 at 10:41:42PM -0400, Mimi Zohar wrote:
On Tue, 2025-09-02 at 12:25 +0800, Coiby Xu wrote:
Currently when both IMA and EVM are in fix mode, the IMA signature will
be reset to IMA hash if a program first stores IMA signature in
security.ima and then sets security.selinux for a file. For example, on
Fedora, after booting the kernel with "ima_appraise=fix evm=fix
ima_policy=appraise_tcb" and installing rpm-plugin-ima, reinstalling a
package will not make good reference IMA signature generated. Instead
IMA hash is generated,
# getfattr -m - -d -e hex /usr/bin/bash
# file: usr/bin/bash
security.ima=0x0404...
This happens because when setting selinux.selinux, the IMA_DIGSIG flag
that had been set early was cleared. As a result, IMA hash is generated
when the file is closed.
Here's a minimal C reproducer,
#include <stdio.h>
#include <sys/xattr.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
int main() {
const char* file_path = "/usr/sbin/test_binary";
const char* hex_string = "030204d33204490066306402304";
int length = strlen(hex_string);
char* ima_attr_value;
int fd;
fd = open(file_path, O_WRONLY|O_CREAT|O_EXCL, 0644);
if (fd == -1) {
perror("Error opening file");
return 1;
}
ima_attr_value = (char*)malloc(length / 2 );
for (int i = 0, j = 0; i < length; i += 2, j++) {
sscanf(hex_string + i, "%2hhx", &ima_attr_value[j]);
}
if (fsetxattr(fd, "security.ima", ima_attr_value, length/2, 0) == -1) {
perror("Error setting extended attribute");
close(fd);
return 1;
}
const char* selinux_value= "system_u:object_r:bin_t:s0";
if (fsetxattr(fd, "security.selinux", selinux_value,
strlen(selinux_value), 0) == -1) {
perror("Error setting extended attribute");
close(fd);
return 1;
}
close(fd);
return 0;
}
Signed-off-by: Coiby Xu <[email protected]>
Thanks, Coiby. Agreed, the ability to clear the IMA_DIGSIG flag should be
limited to security.ima xattr and probably security.evm xattr. Writing other
security xattrs should not affect the IMA_DIGSIG flag.
Thanks for confirming it!
Even without an IMA appraise policy, the security xattrs are written out to the
filesystem, but the IMA_DIGSIG flag is not cached.
It seems I miss some context for the above sentence. If no IMA policy is
configured, no ima_iint_cache will be created. If you mean non-appraisal
policy, will not caching IMA_DIGSIG flag cause any problem?
Please document the tristate values:
0: clear IMA_DIGSIG
1: set IMA_DIGSIG
-1: don't change IMA_DIGSIG
Addressed in v2. Thanks for the suggestion!
---
security/integrity/ima/ima_appraise.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/security/integrity/ima/ima_appraise.c
b/security/integrity/ima/ima_appraise.c
index f435eff4667f..fc82161f8b30 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -708,7 +708,7 @@ static void ima_reset_appraise_flags(struct inode *inode,
int digsig)
set_bit(IMA_CHANGE_XATTR, &iint->atomic_flags);
if (digsig)
set_bit(IMA_DIGSIG, &iint->atomic_flags);
This matches both -1 and 1. Test "digsig == 1" here.
- else
+ else if (digsig != -1)
and test "digsig == 0" here.
clear_bit(IMA_DIGSIG, &iint->atomic_flags);
}
@@ -794,6 +794,8 @@ static int ima_inode_setxattr(struct mnt_idmap *idmap,
struct dentry *dentry,
digsig = (xvalue->type == EVM_IMA_XATTR_DIGSIG);
} else if (!strcmp(xattr_name, XATTR_NAME_EVM) && xattr_value_len > 0) {
digsig = (xvalue->type == EVM_XATTR_PORTABLE_DIGSIG);
+ } else if (result != 1) {
The "if (result != 1)" test is redundant.
I've fixed them in v2. Thanks for reviewing the patch and correcting my
careless mistakes! I'll check if my mind is in a clear thinking state
next time.
thanks,
Mimi
+ digsig = -1;
}
if (result == 1 || evm_revalidate_status(xattr_name)) {
ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
--
Best regards,
Coiby
