I agree with you that it cannot be exploitable on Android, but Kasan is able to find it as OOB if I run syzkaller on x86 based VM image. My last commit is actually only fixing one path, but there are multiple path which are having same issue, so it would be better if fix is given in context_struct_to_string() where length is calculated.
Let me know what is your take on this. From: Stephen Smalley Sent: Monday 13 August, 18:05 Subject: Re: Possible OOB Read in Kernel Heap Memory in call to ext4_xattr_set_entry() To: Sachin Grover, selinux@tycho.nsa.gov, Paul Moore On 08/13/2018 08:23 AM, Stephen Smalley wrote: > On 08/13/2018 01:19 AM, Sachin Grover wrote: >> Hi Stephen/Paul, >> >> This issue was discovered using >> https://android.googlesource.com/kernel/common -b android-4.9-o, but >> I've verified the code path exists in msm-4.4. It likely exists in >> other kernel versions as well. >> >> As a privileged user, one can override the current SELinux context via >> a call to setfscreatecon() or by writing directly to >> /proc/self/attr/fscreate. The context is then used to label the next >> newly created file object, e.g. mknod(). Upon handling the creation of >> the new object on the EXT4 FS we end up calling >> selinux_inode_init_security() to create a new xattr object for the >> inode. This will end up calling context_struct_to_string() to convert >> the SID back to the string version of the SELinux security context >> that was stored. >> >> static int context_struct_to_string(struct context *context, char >> **scontext, u32 *scontext_len) >> { >> char *scontextp; >> >> if (scontext) >> *scontext = NULL; >> *scontext_len = 0; >> >> if (context->len) { >> *scontext_len = context->len; >> if (scontext) { >> *scontext = kstrdup(context->str, GFP_ATOMIC); >> if (!(*scontext)) >> return -ENOMEM; >> } >> return 0; >> } >> >> scontext is populated with the context length, directly from the >> context structure, and using kstrdup() to copy the string. Much later >> down the call stack ext4_xattr_set_entry() is called with the SELinux >> xattr object. >> >> >> if (i->value == EXT4_ZERO_XATTR_VALUE) { >> memset(val, 0, size); >> } else { >> /* Clear the pad bytes first. */ >> memset(val + size - EXT4_XATTR_PAD, 0, >> EXT4_XATTR_PAD); >> memcpy(val, i->value, i->value_len); >> } >> >> SELinux allows for the creation of contexts that include NULL bytes. >> Unfortunately in this case if a NULL byte occurs not as the string >> terminator, then kstrdup() will allocate a memory region smaller than >> what context->len represents. Later in ext4_xattr_set_entry() the >> original context->len will be used in memcpy() and will exceed the >> heap memory allocated by kstrdup() leading to an OOB issue. There may >> also be the potential for memory corruption depending on the total >> length of the SELinux context, but this would require further analysis. >> >> Can you please let me know if my analysis is correct. >> As per me, it looks like there is problem in the code. > > Your earlier commit efe3de79e0b52ca281ef6691480c8c68c82a4657 ensures > that context->len is only ever set to strlen(str)+1. So is it still > possible for this condition to occur? Also, as with the earlier bug, this one is only exploitable for a process with CAP_MAC_ADMIN that is allowed mac_admin permission in SELinux policy, and this is prohibited in Android policy via neverallow and checked by CTS.
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