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Donenfeld" , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-hardening@vger.kernel.org Cc: "Liam R . Howlett" , Yu Zhao , Catalin Marinas , David Hildenbrand , David Howells , Davidlohr Bueso , Matthew Wilcox , SeongJae Park , Sven Schnelle , Will Deacon References: <20221202162724.2009-1-Jason@zx2c4.com> From: Vlastimil Babka In-Reply-To: <20221202162724.2009-1-Jason@zx2c4.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Spamd-Result: default: False [-2.40 / 9.00]; BAYES_HAM(-3.00)[99.99%]; SUBJECT_HAS_UNDERSCORES(1.00)[]; R_DKIM_ALLOW(-0.20)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; R_SPF_ALLOW(-0.20)[+ip4:195.135.220.0/24]; MIME_GOOD(-0.10)[text/plain]; RCVD_NO_TLS_LAST(0.10)[]; FROM_EQ_ENVFROM(0.00)[]; DKIM_TRACE(0.00)[suse.cz:+]; RCPT_COUNT_TWELVE(0.00)[15]; MIME_TRACE(0.00)[0:+]; TO_MATCH_ENVRCPT_SOME(0.00)[]; ARC_SIGNED(0.00)[hostedemail.com:s=arc-20220608:i=1]; MID_RHS_MATCH_FROM(0.00)[]; ARC_NA(0.00)[]; RCVD_COUNT_THREE(0.00)[4]; FROM_HAS_DN(0.00)[]; DMARC_NA(0.00)[suse.cz]; TO_DN_SOME(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[] X-Stat-Signature: st3efzc3yemuiuo3u16eaf4pck48ejut X-Rspamd-Queue-Id: BCC62160018 X-Rspamd-Server: rspam05 X-HE-Tag: 1669998811-389317 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 12/2/22 17:27, Jason A. Donenfeld wrote: > The following program will trigger the BUG_ON that this patch removes, > because the user can munmap() mm->brk: > > #include > #include > #include > #include > > 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: > > 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 > > > Instead, just error out if the original mapping has been removed. > > Fixes: 2e7ce7d354f2 ("mm/mmap: change do_brk_flags() to expand existing VMA and add do_brk_munmap()") That's 6.1-rc1 so probably fast track to Linus as we don't need to introduce such trivial local user DoS in 6.1, right? > Cc: Liam R. Howlett > Cc: Yu Zhao > Cc: Catalin Marinas > Cc: David Hildenbrand > Cc: David Howells > Cc: Davidlohr Bueso > Cc: Matthew Wilcox > Cc: SeongJae Park > Cc: Sven Schnelle > Cc: Vlastimil Babka > Cc: Will Deacon > Cc: Andrew Morton > Signed-off-by: Jason A. Donenfeld Acked-by: Vlastimil Babka > --- > mm/mmap.c | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index c3c5c1d6103d..f5a37fe9a19e 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.