mm: do not BUG_ON missing brk mapping, because userspace can unmap it
The following program will trigger the BUG_ON that this patch removes,
because the user can munmap() mm->brk:
#include <sys/syscall.h>
#include <sys/mman.h>
#include <assert.h>
#include <unistd.h>
static void *brk_now(void)
{
return (void *)syscall(SYS_brk, 0);
}
static void brk_set(void *b)
{
assert(syscall(SYS_brk, b) != -1);
}
int main(int argc, char *argv[])
{
void *b = brk_now();
brk_set(b + 4096);
assert(munmap(b - 4096, 4096 * 2) == 0);
brk_set(b);
return 0;
}
Compile that with musl, since glibc actually uses brk(), and then
execute it, and it'll hit this splat:
kernel BUG at mm/mmap.c:229!
invalid opcode: 0000 [#1] PREEMPT SMP
CPU: 12 PID: 1379 Comm: a.out Tainted: G S U 6.1.0-rc7+ #419
RIP: 0010:__do_sys_brk+0x2fc/0x340
Code: 00 00 4c 89 ef e8 04 d3 fe ff eb 9a be 01 00 00 00 4c 89 ff e8 35 e0 fe ff e9 6e ff ff ff 4d 89 a7 20>
RSP: 0018:
ffff888140bc7eb0 EFLAGS:
00010246
RAX:
0000000000000000 RBX:
00000000007e7000 RCX:
ffff8881020fe000
RDX:
ffff8881020fe001 RSI:
ffff8881955c9b00 RDI:
ffff8881955c9b08
RBP:
0000000000000000 R08:
ffff8881955c9b00 R09:
00007ffc77844000
R10:
0000000000000000 R11:
0000000000000001 R12:
00000000007e8000
R13:
00000000007e8000 R14:
00000000007e7000 R15:
ffff8881020fe000
FS:
0000000000604298(0000) GS:
ffff88901f700000(0000) knlGS:
0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0:
0000000080050033
CR2:
0000000000603fe0 CR3:
000000015ba9a005 CR4:
0000000000770ee0
PKRU:
55555554
Call Trace:
<TASK>
do_syscall_64+0x2b/0x50
entry_SYSCALL_64_after_hwframe+0x46/0xb0
RIP: 0033:0x400678
Code: 10 4c 8d 41 08 4c 89 44 24 10 4c 8b 01 8b 4c 24 08 83 f9 2f 77 0a 4c 8d 4c 24 20 4c 01 c9 eb 05 48 8b>
RSP: 002b:
00007ffc77863890 EFLAGS:
00000212 ORIG_RAX:
000000000000000c
RAX:
ffffffffffffffda RBX:
000000000040031b RCX:
0000000000400678
RDX:
00000000004006a1 RSI:
00000000007e6000 RDI:
00000000007e7000
RBP:
00007ffc77863900 R08:
0000000000000000 R09:
00000000007e6000
R10:
00007ffc77863930 R11:
0000000000000212 R12:
00007ffc77863978
R13:
00007ffc77863988 R14:
0000000000000000 R15:
0000000000000000
</TASK>
Instead, just return the old brk value if the original mapping has been
removed.
[akpm@linux-foundation.org: fix changelog, per Liam]
Link: https://lkml.kernel.org/r/20221202162724.2009-1-Jason@zx2c4.com
Fixes:
2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>