[RFC][PATCH] Security: fix cap_file_mmap() off-by-one error to avoid kernel null pointer exploit
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From: wzt.wzt on 15 Apr 2010 01:50 when addr < dac_mmap_min_addr, cap_file_mmap() will check the process CAP_SYS_RAWIO capability. some code from kernel null pointer exploit: if ((personality(0xffffffff)) != PER_SVR4) { if ((page = mmap(0x0, 0x1000, PROT_READ | PROT_WRITE, MAP_FIXED | MAP_ANONYMOUS| MAP_PRIVATE, 0, 0)) == MAP_FAILED) { perror("mmap"); return -1; } } else { if (mprotect(0x0, 0x1000, PROT_READ | PROT_WRITE | PROT_EXEC) < 0) { perror("mprotect"); return -1; } } printf("[+] Mmap zero memory ok.\n"); [root(a)localhost ~]# echo "1024" > /proc/sys/vm/mmap_min_addr [wzt(a)localhost ~]$ ./exp mmap: Operation not permitted [root(a)localhost ~]# echo "1" > /proc/sys/vm/mmap_min_addr [wzt(a)localhost ~]$ ./exp mmap: Operation not permitted [root(a)localhost ~]# echo "0" > /proc/sys/vm/mmap_min_addr [wzt(a)localhost ~]$ ./exp [+] Mmap zero memory ok. [root(a)localhost ~]# cat /etc/selinux/config ;uname -a SELINUX=enforcing Linux localhost.localdomain 2.6.31.13 #4 SMP Wed Apr 14 17:51:21 CST 2010 i686 i686 i386 GNU/Linux if mmap_min_addr is equal 0, whether the process has the CAP_SYS_RAWIO capability or not, it can mmap zero memory. The administrator set dac_mmap_min_addr as 0 for some reason, the kernel null pointer bugs will be exploited again. when dac_mmap_min_addr equal 1, cap_file_mmap() will check it, but dac_mmap_min_addr equal 0, it not check it though the process not has the CAP_SYS_RAWIO capability. when kernel null pointer bug happens, eip is below PAGE_SIZE, that means if eip=0x00000001 for example, and dac_mmap_min_addr=0, user process can mmap zero memory. *(char *)0 = '\x90'; *(char *)1 = '\x90'; *(char *)2 = '\xe9'; *(unsigned long *)3 = (unsigned long)&exploit_code - 7; the kernel null pointer bug can be exploited. So if the process not has the CAP_SYS_RAWIO capability, though the dac_mmap_min_addr is equal 0, it will not be mmapd in zero memory. Also fix the comment of cap_file_mmap(). Signed-off-by: Zhitong Wang <zhitong.wangzt(a)alibaba-inc.com> --- security/commoncap.c | 4 ++-- 1 files changed, 2 insertions(+), 2 deletions(-) diff --git a/security/commoncap.c b/security/commoncap.c index 6166973..cc6b458 100644 --- a/security/commoncap.c +++ b/security/commoncap.c @@ -931,7 +931,7 @@ int cap_vm_enough_memory(struct mm_struct *mm, long pages) * @addr: address attempting to be mapped * @addr_only: unused * - * If the process is attempting to map memory below mmap_min_addr they need + * If the process is attempting to map memory below dac_mmap_min_addr they need * CAP_SYS_RAWIO. The other parameters to this function are unused by the * capability security module. Returns 0 if this mapping should be allowed * -EPERM if not. @@ -942,7 +942,7 @@ int cap_file_mmap(struct file *file, unsigned long reqprot, { int ret = 0; - if (addr < dac_mmap_min_addr) { + if (addr <= dac_mmap_min_addr) { ret = cap_capable(current, current_cred(), CAP_SYS_RAWIO, SECURITY_CAP_AUDIT); /* set PF_SUPERPRIV if it turns out we allow the low mmap */ -- 1.6.5.3 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |