On 12/02/2017 02:19 PM, Matthew Wilcox wrote: > On Sat, Dec 02, 2017 at 07:49:20PM +0100, Jann Horn wrote: >> On Sat, Dec 2, 2017 at 4:05 PM, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: >>> On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@xxxxxxxxx wrote: [...] > > Maybe that should be up front rather than buried at the end of the sentence. > > "In a multi-threaded process, the address space can change in response to > virtually any library call. This is because almost any library call may be > implemented by using dlopen(3) to load another shared library, which will be > mapped into the process's address space. The PAM libraries are an excellent > example, as well as more obvious examples like brk(2), malloc(3) and even > pthread_create(3)." > > What do you think? > Hi Matthew, Here is a new version, based on your and Jann's comments. I also added a reference to MAP_FIXED_SAFE. If it looks close, I'll send a v2 with proper formatting applied. I did wonder briefly if your ATM reference was a oblique commentary about security, but then realized...you probably just needed some cash. :) ----- This option is extremely hazardous (when used on its own) and moderately non-portable. On portability: a process's memory map may change significantly from one run to the next, depending on library versions, kernel versions and ran‐ dom numbers. On hazards: this option forcibly removes pre-existing mappings, making it easy for a multi-threaded process to corrupt its own address space. For example, thread A looks through /proc/<pid>/maps and locates an available address range, while thread B simultaneously acquires part or all of that same address range. Thread A then calls mmap(MAP_FIXED), effectively overwriting thread B's mapping. Thread B need not create a mapping directly; simply making a library call that, internally, uses dlopen(3) to load some other shared library, will suffice. The dlopen(3) call will map the library into the process's address space. Furthermore, almost any library call may be implemented using this technique. Examples include brk(2), malloc(3), pthread_cre‐ ate(3), and the PAM libraries (http://www.linux-pam.org). Given the above limitations, one of the very few ways to use this option safely is: mmap() a region, without specifying MAP_FIXED. Then, within that region, call mmap(MAP_FIXED) to suballocate regions. This avoids both the portability problem (because the first mmap call lets the ker‐ nel pick the address), and the address space corruption problem (because the region being overwritten is already owned by the calling thread). Newer kernels (Linux 4.16 and later) have a MAP_FIXED_SAFE option that avoids the corruption problem; if available, MAP_FIXED_SAFE should be preferred over MAP_FIXED. thanks, John Hubbard NVIDIA
