Date: Sat, 10 Jun 2017 11:01:18 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: a possible need for MAP_FIXED in ldso/dynlink.c ? On Sat, Jun 10, 2017 at 04:32:25PM +0200, u-uy74@...ey.se wrote: > On Sat, Jun 10, 2017 at 08:26:10AM -0400, Rich Felker wrote: > > On Sat, Jun 10, 2017 at 12:51:51PM +0200, u-uy74@...ey.se wrote: > > > Adding the MAP_FIXED flag, both conditionally or not, seems to work > > > around the particular problem but I am unsure about all its implications > > > and consequences, among others under the current Linux implementation > > > of the Lunux ABI. > > > Use of MAP_FIXED with a memory range you don't already own is an > > invalid and unsafe operation. You may end up mapping over top of > > yourself, even. > > But the latter should be possible to avoid as long as we know where > ourself is located (?) Or do we? Not easily. And you don't know that something else isn't mapped there, like perhaps a data page needed by the vdso or some other kernel-mapped code (perhaps sigreturn trampolines). The only way to know that would be nasty hacks like parsing /proc/self/maps. And the code that does the mapping is not restricted to running before program startup. At any later time, any check is subject to TOCTOU races. Use of MAP_FIXED is simply not safe/not valid except when the caller already "owns" the address range to be mapped. It's not sufficient to require that "nothing else be using it"; you really need "I own it and I authorize it to be replaced with something else". > > Implementations should honor the requested address > > passed to mmap and only fail to provide it if it's already in use. > > I did not see this statement anywhere in the mmap() documentation > (and even if I am misinformed, you say "should" not "must"). > > E.g. http://pubs.opengroup.org/onlinepubs/9699919799/functions/mmap.html > says > "When MAP_FIXED is not set, the implementation uses addr in an > implementation-defined manner to arrive at pa." Indeed, not all implementations will define this in a way that admits mapping programs at a hard-coded address, but then they're not suitable for dynamic loading of non-PIE programs. BTW one reason the standard can't really formally define loading at a requested address is that pointers that don't point to an existing object or null are not even a valid concept in the C language. > > applications that don't have a reason for requesting a > > particular address should, and do, pass 0 as the request. > > Notwithstanding this, I find the API very unclear about how to > safely check whether the use of a certain address range is impossible > or just differs from the particular "implementation-defined manner" > to derive the returned value. > > Does this mean that musl can not implement the explicit dynamic loader > with mere Posix mmap(), only with the "more tightly specified Linux mmap()"? Yes, I think that's accurate. Of course you can use PIE binaries and then you don't have to care about it, since they can be loaded at any address. Rich
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