Re: [PATCHv4 4/5] x86/mm: check in_compat_syscall() instead TIF_ADDR32 for mmap(MAP_32BIT)

From: Dmitry Safonov
Date: Tue Feb 14 2017 - 11:15:40 EST

On 02/11/2017 11:13 PM, Thomas Gleixner wrote:
On Mon, 30 Jan 2017, Dmitry Safonov wrote:

At this momet, logic in arch_get_unmapped_area{,_topdown} for mmaps with
MAP_32BIT flag checks TIF_ADDR32 which means:
o if 32-bit ELF changes mode to 64-bit on x86_64 and then tries to
mmap() with MAP_32BIT it'll result in addr over 4Gb (as default is
top-down allocation)
o if 64-bit ELF changes mode to 32-bit and tries mmap() with MAP_32BIT,
it'll allocate only memory in 1GB space: [0x40000000, 0x80000000).

Fix it by handeling MAP_32BIT in 64-bit syscalls only.

I really have a hard time to understand what is fixed and how that is
related to the $subject.

Again. Please explain the problem first properly so one can understand the
issue immediately.

Ok, rewrote the changes log.

As a little bonus it'll make thread flag a little less used.

I really do not understand the bonus part here. You replace the thread flag
check with a different one and AFAICT this looks like oart of the 'fix'.

It's a part of the fix, right.
What I meant here is that after those patches TIF_ADDR32 is no more
used after exec() time. That's bonus as if we manage to change exec()
code in some way (i.e., pass address restriction as a parameter), we'll
have additional free thread info flag.



@@ -101,7 +101,7 @@ SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len,
static void find_start_end(unsigned long flags, unsigned long *begin,
unsigned long *end)
- if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT)) {
+ if (!in_compat_syscall() && (flags & MAP_32BIT)) {
/* This is usually used needed to map code in small
model, so it needs to be in the first 31bit. Limit
it to that. This means we need to move the
@@ -195,7 +195,7 @@ arch_get_unmapped_area_topdown(struct file *filp, const unsigned long addr0,
return addr;

/* for MAP_32BIT mappings we force the legacy mmap base */
- if (!test_thread_flag(TIF_ADDR32) && (flags & MAP_32BIT))
+ if (!in_compat_syscall() && (flags & MAP_32BIT))
goto bottomup;

/* requesting a specific address */