On 5/6/21 9:55 AM, Warner Losh wrote: >>> Where is this freed? Also, alloca just moves a stack pointer, where >> malloc >>> has complex interactions. Are you sure that's a safe change here? >> >> It's freed any time the g_autofree variable goes out of scope (that's >> what the g_autofree macro is for). Yes, the change is safe, although >> you are right that switching to malloc is going to be a bit more >> heavyweight than what alloca used. What's more, it adds safety: if >> count was under user control, a user could pass a value that could cause >> alloca to allocate more than 4k and accidentally mess up stack guard >> pages, while malloc() uses the heap and therefore cannot cause stack bugs. >> > > I'm not sure I understand that argument, since we're not compiling bsd-user > with the stack-smash-protection stuff enabled, so there's no guard pages > involved. The stack can grow quite large and the unmapped page at > the end of the stack would catch any overflows. Since these allocations > are on the top of the stack, they won't overflow into any other frames > and subsequent calls are made with them already in place. With alloca() on user-controlled size, the user can set up the size to be larger than the unmapped guard page, at which point you CANNOT catch the stack overflow because the alloca can skip the guard page and wrap into other valid memory. Compiling with stack-smash-protection stuff enabled will catch such a bad alloca(); but the issue at hand here is not when stack-smash-protection is enabled, but the more common case when it is disabled (at which point the only protection you have is the guard page, but improper use of alloca() can bypass the guard page). Not all alloca() arguments are under user control, but it is easier as a matter of policy to blindly avoid alloca() than it is to audit which calls have safe sizes and therefore will not risk user control bypassing stack guards. > > malloc, on the other hand, involves taking out a number of mutexes > and similar things to obtain the memory, which may not necessarily > be safe in all the contexts system calls can be called from. System > calls are, typically, async safe and can be called from signal handlers. > alloca() is async safe, while malloc() is not. So changing the calls > from alloca to malloc makes calls to system calls in signal handlers > unsafe and potentially introducing buggy behavior as a result. Correct, use of malloc() is not safe within signal handlers. But these calls are not within signal handlers - or am _I_ missing something? Is the point of *-user code to emulate syscalls that are reachable from code installed in a signal handler, at which point introducing an async-unsafe call to malloc in our emulation is indeed putting the application at risk of a malloc deadlock? Ultimately, we're trading one maintenance headache (determining which alloca() size calls might be under user control) for another (determining that malloca() calls are not in a signal context), but the latter is far more common such that we can use existing tooling to make that conversion safely (both in the fact that the compiler has flags to warn about alloca() usage, and in the fact that Coverity is good at flagging improper uses of malloc() such as within a function reachable from something installed in a signal handler). But I'm not familiar enough with the bsd/linux-user code to know if your point about having to use only async-safe functionalities is a valid limitation on our emulation. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
