In the Linux kernel, the following vulnerability has been resolved:
ima: don't clear IMA_DIGSIG flag when setting or removing non-IMA xattr
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 writes/removes some other security xattr for the file.
For example, on Fedora, after booting the kernel with "ima_appraise=fix evm=fix ima_policy=appraise_tcb" and installing rpm-plugin-ima, installing/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 security.selinux, the IMA_DIGSIG flag that had been set early was cleared. As a result, IMA hash is generated when the file is closed.
Similarly, IMA signature can be cleared on file close after removing security xattr like security.evm or setting/removing ACL.
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 which sets security.selinux as the last step which can also replaced by removing security.evm or setting ACL,
#include #include #include #include #include #include
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; }