On 17/07/2024 07:33, Jeff Xu wrote:

Consider those cases: I think:
a> relying purely on userspace for enforcement does't seem to be
effective,  e.g. it is trivial  to call open(), then mmap() it into
executable memory.

If there's a way to do this without running executable code that had to 
pass a previous execveat() check, then yeah, it's not effective (e.g. a 
Python interpreter that *doesn't* enforce execveat() is a trivial way to 
do it).

Once arbitrary code is running, all bets are off. So long as all 
arbitrary code is being checked itself, it's allowed to do things that 
would bypass later checks (and it's up to whoever audited it in the 
first place to prevent this by not giving it the special mark that 
allows it to pass the check).

b> if both user space and kernel need to call AT_CHECK, the faccessat
seems to be a better place for AT_CHECK, e.g. kernel can call
do_faccessat(AT_CHECK) and userspace can call faccessat(). This will
avoid complicating the execveat() code path.

What do you think ?

Thanks
-Jeff