Why can’t I mmap(MAP_FIXED) the highest virtual page in a 32-bit Linux process on a 64-bit kernel?

While attempting to test Is it allowed to access memory that spans the zero boundary in x86? in user-space on Linux, I wrote a 32-bit test program that tries to map the low and high pages of 32-bit virtual address space.

After echo 0 | sudo tee /proc/sys/vm/mmap_min_addr, I can map the zero page, but I don’t know why I can’t map -4096, i.e. (void*)0xfffff000, the highest page. Why does mmap2((void*)-4096) return -ENOMEM?

strace ./a.out 
execve("./a.out", ["./a.out"], 0x7ffe08827c10 /* 65 vars */) = 0
strace: [ Process PID=1407 runs in 32 bit mode. ]
....
mmap2(0xfffff000, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = -1 ENOMEM (Cannot allocate memory)
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0

Also, what check is rejecting it in linux/mm/mmap.c, and why is it designed that way? Is this part of making sure that creating a pointer to one-past-an-object doesn’t wrap around and break pointer comparisons, because ISO C and C++ allow creating a pointer to one-past-the-end, but otherwise not outside of objects.

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I’m running under a 64-bit kernel (4.12.8-2-ARCH on Arch Linux), so 32-bit user space has the entire 4GiB available. (Unlike 64-bit code on a 64-bit kernel, or with a 32-bit kernel where the 2:2 or 3:1 user/kernel split would make the high page a kernel address.)

I haven’t tried from a minimal static executable (no CRT startup or libc, just asm) because I don’t think that would make a difference. None of the CRT startup system calls look suspicious.

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While stopped at a breakpoint, I checked /proc/PID/maps. The top page isn’t already in use. The stack includes the 2nd highest page, but the top page is unmapped.

00000000-00001000 rw-p 00000000 00:00 0             ### the mmap(0) result
08048000-08049000 r-xp 00000000 00:15 3120510                 /home/peter/src/SO/a.out
08049000-0804a000 r--p 00000000 00:15 3120510                 /home/peter/src/SO/a.out
0804a000-0804b000 rw-p 00001000 00:15 3120510                 /home/peter/src/SO/a.out
f7d81000-f7f3a000 r-xp 00000000 00:15 1511498                 /usr/lib32/libc-2.25.so
f7f3a000-f7f3c000 r--p 001b8000 00:15 1511498                 /usr/lib32/libc-2.25.so
f7f3c000-f7f3d000 rw-p 001ba000 00:15 1511498                 /usr/lib32/libc-2.25.so
f7f3d000-f7f40000 rw-p 00000000 00:00 0 
f7f7c000-f7f7e000 rw-p 00000000 00:00 0 
f7f7e000-f7f81000 r--p 00000000 00:00 0                       [vvar]
f7f81000-f7f83000 r-xp 00000000 00:00 0                       [vdso]
f7f83000-f7fa6000 r-xp 00000000 00:15 1511499                 /usr/lib32/ld-2.25.so
f7fa6000-f7fa7000 r--p 00022000 00:15 1511499                 /usr/lib32/ld-2.25.so
f7fa7000-f7fa8000 rw-p 00023000 00:15 1511499                 /usr/lib32/ld-2.25.so
fffdd000-ffffe000 rw-p 00000000 00:00 0                       [stack]

Are there VMA regions that don’t show up in maps that still convince the kernel to reject the address? I looked at the occurrences of ENOMEM in linux/mm/mmapc., but it’s a lot of code to read so maybe I missed something. Something that reserves some range of high addresses, or because it’s next to the stack?

Making the system calls in the other order doesn’t help (but PAGE_ALIGN and similar macros are written carefully to avoid wrapping around before masking, so that wasn’t likely anyway.)

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Full source, compiled with gcc -O3 -fno-pie -no-pie -m32 address-wrap.c:

#include <sys/mman.h>

//void *mmap(void *addr, size_t len, int prot, int flags,
//           int fildes, off_t off);

int main(void) {
    volatile unsigned *high =
        mmap((void*)-4096L, 4096, PROT_READ | PROT_WRITE,
             MAP_FIXED|MAP_PRIVATE|MAP_ANONYMOUS,
             -1, 0);
    volatile unsigned *zeropage =
        mmap((void*)0, 4096, PROT_READ | PROT_WRITE,
             MAP_FIXED|MAP_PRIVATE|MAP_ANONYMOUS,
             -1, 0);


    return (high == MAP_FAILED) ? 2 : *high;
}

(I left out the part that tried to deref (int*)-2 because it just segfaults when mmap fails.)

Answer

The mmap function eventually calls either do_mmap or do_brk_flags which do the actual work of satisfying the memory allocation request. These functions in turn call get_unmapped_area. It is in that function that the checks are made to ensure that memory cannot be allocated beyond the user address space limit, which is defined by TASK_SIZE. I quote from the code:

 * There are a few constraints that determine this:
 *
 * On Intel CPUs, if a SYSCALL instruction is at the highest canonical
 * address, then that syscall will enter the kernel with a
 * non-canonical return address, and SYSRET will explode dangerously.
 * We avoid this particular problem by preventing anything executable
 * from being mapped at the maximum canonical address.
 *
 * On AMD CPUs in the Ryzen family, there's a nasty bug in which the
 * CPUs malfunction if they execute code from the highest canonical page.
 * They'll speculate right off the end of the canonical space, and
 * bad things happen.  This is worked around in the same way as the
 * Intel problem.

#define TASK_SIZE_MAX   ((1UL << __VIRTUAL_MASK_SHIFT) - PAGE_SIZE)

#define IA32_PAGE_OFFSET    ((current->personality & ADDR_LIMIT_3GB) ? 
                    0xc0000000 : 0xFFFFe000)

#define TASK_SIZE       (test_thread_flag(TIF_ADDR32) ? 
IA32_PAGE_OFFSET : TASK_SIZE_MAX)

On processors with 48-bit virtual address spaces, __VIRTUAL_MASK_SHIFT is 47.

Note that TASK_SIZE is specified depending on whether the current process is 32-bit on 32-bit, 32-bit on 64-bit, 64-bit on 64-bit. For 32-bit processes, two pages are reserved; one for the vsyscall page and the other used as a guard page. Essentially, the vsyscall page cannot be unmapped and so the highest address of the user address space is effectively 0xFFFFe000. For 64-bit processes, one guard page is reserved. These pages are only reserved on 64-bit Intel and AMD processors because only on these processors the SYSCALL mechanism is used.

Here is the check that is performed in get_unmapped_area:

if (addr > TASK_SIZE - len)
     return -ENOMEM;