On Mon, 2025-09-08 at 18:58 +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.
The problem description should be generic.
-> and then writes some other security xattr for the file.
Start a new paragraph here for the example.
> 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.
Start a new paragraph here, adding a sentence describing the solution to the
problem. For example,
Prevent replacing the IMA file signature with a file hash, by preventing the
IMA_DIGSIG flag from being reset.
>
> 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 <c...@redhat.com>
Thanks, Coiby. The updated patch looks good. Have you looked at the other
calls to ima_reset_appraise_flags() to make sure they don't need to be adjusted?
thanks,
Mimi
> ---
> security/integrity/ima/ima_appraise.c | 15 +++++++++++++--
> 1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/security/integrity/ima/ima_appraise.c
> b/security/integrity/ima/ima_appraise.c
> index f435eff4667f..4e4750ea41ad 100644
> --- a/security/integrity/ima/ima_appraise.c
> +++ b/security/integrity/ima/ima_appraise.c
> @@ -694,6 +694,15 @@ static int ima_protect_xattr(struct dentry *dentry,
> const char *xattr_name,
> return 0;
> }
>
> +/*
> + * ima_reset_appraise_flags - reset ima_iint_cache flags
> + *
> + * @digsig: whether to clear/set IMA_DIGSIG flag, tristate values
> + * 0: clear IMA_DIGSIG
> + * 1: set IMA_DIGSIG
> + * -1: don't change IMA_DIGSIG
> + *
> + */
> static void ima_reset_appraise_flags(struct inode *inode, int digsig)
> {
> struct ima_iint_cache *iint;
> @@ -706,9 +715,9 @@ static void ima_reset_appraise_flags(struct inode *inode,
> int digsig)
> return;
> iint->measured_pcrs = 0;
> set_bit(IMA_CHANGE_XATTR, &iint->atomic_flags);
> - if (digsig)
> + if (digsig == 1)
> set_bit(IMA_DIGSIG, &iint->atomic_flags);
> - else
> + else if (digsig == 0)
> clear_bit(IMA_DIGSIG, &iint->atomic_flags);
> }
>
> @@ -794,6 +803,8 @@ 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 {
> + digsig = -1;
> }
> if (result == 1 || evm_revalidate_status(xattr_name)) {
> ima_reset_appraise_flags(d_backing_inode(dentry), digsig);
