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authorJason A. Donenfeld <Jason@zx2c4.com>2022-12-02 17:27:24 +0100
committerAndrew Morton <akpm@linux-foundation.org>2022-12-09 18:41:16 -0800
commitf5ad5083404bb56c9de777dccb68c6672ef6487e (patch)
tree72175fd03301eaa859ef13c88e3a85f13f6f02eb /mm/mmap.c
parent38f1d4aefdac30600698ab07cb1e41fca5851f7a (diff)
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>
Diffstat (limited to 'mm/mmap.c')
-rw-r--r--mm/mmap.c3
1 files changed, 1 insertions, 2 deletions
diff --git a/mm/mmap.c b/mm/mmap.c
index 74a84eb33b90..0024e615d069 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -226,8 +226,7 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
/* Search one past newbrk */
mas_set(&mas, newbrk);
brkvma = mas_find(&mas, oldbrk);
- BUG_ON(brkvma == NULL);
- if (brkvma->vm_start >= oldbrk)
+ if (!brkvma || brkvma->vm_start >= oldbrk)
goto out; /* mapping intersects with an existing non-brk vma. */
/*
* mm->brk must be protected by write mmap_lock.