From: Rustam Kovhaev rkovhaev@gmail.com
[ Upstream commit ccff81e1d028bbbf8573d3364a87542386c707bf ]
kmemleak scans struct page, but it does not scan the page content. If we allocate some memory with kmalloc(), then allocate page with alloc_page(), and if we put kmalloc pointer somewhere inside that page, kmemleak will report kmalloc pointer as a false positive.
We can instruct kmemleak to scan the memory area by calling kmemleak_alloc() and kmemleak_free(), but part of struct bpf_ringbuf is mmaped to user space, and if struct bpf_ringbuf changes we would have to revisit and review size argument in kmemleak_alloc(), because we do not want kmemleak to scan the user space memory. Let's simplify things and use kmemleak_not_leak() here.
For posterity, also adding additional prior analysis from Andrii:
I think either kmemleak or syzbot are misreporting this. I've added a bunch of printks around all allocations performed by BPF ringbuf. [...] On repro side I get these two warnings:
[vmuser@archvm bpf]$ sudo ./repro BUG: memory leak unreferenced object 0xffff88810d538c00 (size 64): comm "repro", pid 2140, jiffies 4294692933 (age 14.540s) hex dump (first 32 bytes): 00 af 19 04 00 ea ff ff c0 ae 19 04 00 ea ff ff ................ 80 ae 19 04 00 ea ff ff c0 29 2e 04 00 ea ff ff .........)...... backtrace: [<0000000077bfbfbd>] __bpf_map_area_alloc+0x31/0xc0 [<00000000587fa522>] ringbuf_map_alloc.cold.4+0x48/0x218 [<0000000044d49e96>] __do_sys_bpf+0x359/0x1d90 [<00000000f601d565>] do_syscall_64+0x2d/0x40 [<0000000043d3112a>] entry_SYSCALL_64_after_hwframe+0x44/0xae
BUG: memory leak unreferenced object 0xffff88810d538c80 (size 64): comm "repro", pid 2143, jiffies 4294699025 (age 8.448s) hex dump (first 32 bytes): 80 aa 19 04 00 ea ff ff 00 ab 19 04 00 ea ff ff ................ c0 ab 19 04 00 ea ff ff 80 44 28 04 00 ea ff ff .........D(..... backtrace: [<0000000077bfbfbd>] __bpf_map_area_alloc+0x31/0xc0 [<00000000587fa522>] ringbuf_map_alloc.cold.4+0x48/0x218 [<0000000044d49e96>] __do_sys_bpf+0x359/0x1d90 [<00000000f601d565>] do_syscall_64+0x2d/0x40 [<0000000043d3112a>] entry_SYSCALL_64_after_hwframe+0x44/0xae
Note that both reported leaks (ffff88810d538c80 and ffff88810d538c00) correspond to pages array bpf_ringbuf is allocating and tracking properly internally. Note also that syzbot repro doesn't close FD of created BPF ringbufs, and even when ./repro itself exits with error, there are still two forked processes hanging around in my system. So clearly ringbuf maps are alive at that point. So reporting any memory leak looks weird at that point, because that memory is being used by active referenced BPF ringbuf.
It's also a question why repro doesn't clean up its forks. But if I do a `pkill repro`, I do see that all the allocated memory is /properly/ cleaned up [and the] "leaks" are deallocated properly.
BTW, if I add close() right after bpf() syscall in syzbot repro, I see that everything is immediately deallocated, like designed. And no memory leak is reported. So I don't think the problem is anywhere in bpf_ringbuf code, rather in the leak detection and/or repro itself.
Reported-by: syzbot+5d895828587f49e7fe9b@syzkaller.appspotmail.com Signed-off-by: Rustam Kovhaev rkovhaev@gmail.com [ Daniel: also included analysis from Andrii to the commit log ] Signed-off-by: Daniel Borkmann daniel@iogearbox.net Tested-by: syzbot+5d895828587f49e7fe9b@syzkaller.appspotmail.com Cc: Dmitry Vyukov dvyukov@google.com Cc: Andrii Nakryiko andrii@kernel.org Link: https://lore.kernel.org/bpf/CAEf4BzYk+dqs+jwu6VKXP-RttcTEGFe+ySTGWT9CRNkagDi... Link: https://lore.kernel.org/lkml/YNTAqiE7CWJhOK2M@nuc10 Link: https://lore.kernel.org/lkml/20210615101515.GC26027@arm.com Link: https://syzkaller.appspot.com/bug?extid=5d895828587f49e7fe9b Link: https://lore.kernel.org/bpf/20210626181156.1873604-1-rkovhaev@gmail.com Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/bpf/ringbuf.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/kernel/bpf/ringbuf.c b/kernel/bpf/ringbuf.c index 84b3b35fc0d0..9e0c10c6892a 100644 --- a/kernel/bpf/ringbuf.c +++ b/kernel/bpf/ringbuf.c @@ -8,6 +8,7 @@ #include <linux/vmalloc.h> #include <linux/wait.h> #include <linux/poll.h> +#include <linux/kmemleak.h> #include <uapi/linux/btf.h>
#define RINGBUF_CREATE_FLAG_MASK (BPF_F_NUMA_NODE) @@ -105,6 +106,7 @@ static struct bpf_ringbuf *bpf_ringbuf_area_alloc(size_t data_sz, int numa_node) rb = vmap(pages, nr_meta_pages + 2 * nr_data_pages, VM_ALLOC | VM_USERMAP, PAGE_KERNEL); if (rb) { + kmemleak_not_leak(pages); rb->pages = pages; rb->nr_pages = nr_pages; return rb;