On Sat, Jul 31, 2021 at 08:20:29PM +0100, Sudip Mukherjee wrote: > Hi All, > > We had been running syzkaller on v5.10.y and a "memory leak in > do_seccomp" was being reported on it. I got some time to check that > today and have managed to get a syzkaller > reproducer. I dont have a C reproducer which I can share but I can use > the syz-reproducer to reproduce this with next-20210730. > The old report on v5.10.y is at > https://elisa-builder-00.iol.unh.edu/syzkaller/report?id=f6ddd3b592f00e95f9cbd2e74f70a5b04b015c6f Thanks for the details! Is this the same as what syzbot saw here (with a C reproducer)? https://syzkaller.appspot.com/bug?id=2809bb0ac77ad9aa3f4afe42d6a610aba594a987 I can't figure out what happened with the "Patch testing request" that was made; there's no link? > > BUG: memory leak > unreferenced object 0xffff888019282c00 (size 512): > comm "syz-executor.1", pid 7389, jiffies 4294761829 (age 17.841s) > hex dump (first 32 bytes): > 01 00 00 00 01 00 00 00 00 00 00 00 00 00 00 00 ................ > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ > backtrace: > [<00000000762c0963>] do_seccomp+0x2d5/0x27d0 Can you run "./scripts/faddr2line do_seccomp+0x2d5/0x27d0" for this? I expect it'll be: sfilter = kzalloc(sizeof(*sfilter), GFP_KERNEL | __GFP_NOWARN); > [<0000000006e512d1>] do_syscall_64+0x3b/0x90 > [<0000000094ae9ff8>] entry_SYSCALL_64_after_hwframe+0x44/0xae The "size 512" in your v5.10.y report is from seccomp_prepare_filter() (noted above). seccomp_prepare_filter() cleans up its error paths. > > BUG: memory leak > unreferenced object 0xffffc900006b5000 (size 4096): > comm "syz-executor.1", pid 7389, jiffies 4294761829 (age 17.841s) > hex dump (first 32 bytes): > 01 00 00 00 00 00 00 00 00 00 00 00 05 00 00 00 ................ > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ > backtrace: > [<00000000854901e5>] __vmalloc_node_range+0x550/0x9a0 > [<000000002686628f>] __vmalloc_node+0xb5/0x100 > [<0000000004cbd298>] bpf_prog_alloc_no_stats+0x38/0x350 > [<0000000009149728>] bpf_prog_alloc+0x24/0x170 > [<000000000fe7f1e7>] bpf_prog_create_from_user+0xad/0x2e0 > [<000000000c70eb02>] do_seccomp+0x325/0x27d0 > [<0000000006e512d1>] do_syscall_64+0x3b/0x90 > [<0000000094ae9ff8>] entry_SYSCALL_64_after_hwframe+0x44/0xae Again, I'm curious about where do_seccomp+0x325/0x27d0 is for this, but the matching one in v5.10 shows: ret = bpf_prog_create_from_user(&sfilter->prog, fprog, seccomp_check_filter, save_orig); This and everything remaining below else has bpf_prog_create_from_user() in the allocation path. > > BUG: memory leak > unreferenced object 0xffff888026eb1000 (size 2048): > comm "syz-executor.1", pid 7389, jiffies 4294761829 (age 17.842s) > hex dump (first 32 bytes): > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ > 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................ > backtrace: > [<0000000072de7240>] bpf_prog_alloc_no_stats+0xeb/0x350 > [<0000000009149728>] bpf_prog_alloc+0x24/0x170 > [<000000000fe7f1e7>] bpf_prog_create_from_user+0xad/0x2e0 > [<000000000c70eb02>] do_seccomp+0x325/0x27d0 > [<0000000006e512d1>] do_syscall_64+0x3b/0x90 > [<0000000094ae9ff8>] entry_SYSCALL_64_after_hwframe+0x44/0xae > > BUG: memory leak > unreferenced object 0xffff888014dddac0 (size 16): > comm "syz-executor.1", pid 7389, jiffies 4294761829 (age 17.842s) > hex dump (first 16 bytes): > 01 00 ca 08 80 88 ff ff c8 ef df 14 80 88 ff ff ................ These are two kernel pointers: 0xffff888008ca0001 (unaligned by 1 byte?!) 0xffff888014dfefc8 Ah, no, this is from: struct sock_fprog_kern { u16 len; struct sock_filter *filter; }; The "ca 08 80 88 ff ff" bytes are uninitialized padding. ;) "len" has a value of 1 (which matches the syzkaller reproducer args below of a single BPF instruction). fp->orig_prog = kmalloc(sizeof(*fkprog), GFP_KERNEL); if (!fp->orig_prog) return -ENOMEM; fkprog = fp->orig_prog; fkprog->len = fprog->len; ... fkprog->filter = kmemdup(fp->insns, fsize, GFP_KERNEL | __GFP_NOWARN); > backtrace: > [<00000000c5d4ed93>] bpf_prog_store_orig_filter+0x7b/0x1e0 > [<000000007cb21c2a>] bpf_prog_create_from_user+0x1c6/0x2e0 > [<000000000c70eb02>] do_seccomp+0x325/0x27d0 > [<0000000006e512d1>] do_syscall_64+0x3b/0x90 > [<0000000094ae9ff8>] entry_SYSCALL_64_after_hwframe+0x44/0xae > > BUG: memory leak > unreferenced object 0xffff888014dfefc8 (size 8): > comm "syz-executor.1", pid 7389, jiffies 4294761829 (age 17.842s) > hex dump (first 8 bytes): > 06 00 00 00 ff ff ff 7f ........ This contains a userspace (likely stack) pointer, and is referenced by the second pointer above. (i.e. kmemdup() above, but how have the contents become a user stack pointer?) > backtrace: > [<00000000ee5550f8>] kmemdup+0x23/0x50 > [<00000000f1acd067>] bpf_prog_store_orig_filter+0x103/0x1e0 > [<000000007cb21c2a>] bpf_prog_create_from_user+0x1c6/0x2e0 > [<000000000c70eb02>] do_seccomp+0x325/0x27d0 > [<0000000006e512d1>] do_syscall_64+0x3b/0x90 > [<0000000094ae9ff8>] entry_SYSCALL_64_after_hwframe+0x44/0xae > > Not sure if this has been already reported or not, but I will be happy > to test if you have a fix for this. I was suspecting a missing error path free near bpf_prepare_filter() as called by bpf_prog_create_from_user() here: /* bpf_prepare_filter() already takes care of freeing * memory in case something goes wrong. */ fp = bpf_prepare_filter(fp, trans); if (IS_ERR(fp)) return PTR_ERR(fp); Since only seccomp and af_packet use bpf_prog_create_from_user(), and af_packet sets neither a "trans" callback nor save_orig. But if "trans" fails (due to some BPF instructions seccomp doesn't support), I'd expect this leak to be detected more often. bpf_prepare_filter() is documented as cleaning up allocations on failure, though I notice its cleanup differs from bpf_prog_create_from_user()'s, which uses __bpf_prog_free() instead of __bfp_prog_release(). But that should only make a difference for orig_prog getting freed, and bpf_prog_store_orig_filter() should already be freeing that on failures too. Similarly, bpf_migrate_filter() cleanups up on failure too, so this doesn't seem to be it: if (!fp->jited) fp = bpf_migrate_filter(fp); return fp; So, I'm going to assume the missing free is somehow related to process management, since I see the Syzkaller reproducer mentions SECCOMP_SET_MODE_FILTER_LISTENER, fork(), and ptrace(). :) Quoting from the v5.10.y report: > # {Threaded:true Collide:true Repeat:true RepeatTimes:0 Procs:8 Slowdown:1 Sandbox:none Fault:false FaultCall:-1 FaultNth:0 Leak:true NetInjection:true NetDevices:true NetReset:true Cgroups:true BinfmtMisc:true CloseFDs:true KCSAN:false DevlinkPCI:false USB:true VhciInjection:false Wifi:false IEEE802154:false Sysctl:true UseTmpDir:true HandleSegv:true Repro:false Trace:false} > seccomp$SECCOMP_SET_MODE_FILTER_LISTENER(0x1, 0x0, &(0x7f0000000000)={0x1, &(0x7f0000000040)=[{0x6, 0x0, 0x0, 0x7fffffff}]}) 0x1 is SECCOMP_SET_MODE_FILTER 0x0 is empty flags {0x6, 0x0, 0x0, 0x7fffffff} is BPF_STMT(BPF_RET, SECCOMP_RET_ALLOW | 0xffff) For "SECCOMP_SET_MODE_FILTER_LISTENER", defined here: https://github.com/google/syzkaller/blob/master/sys/linux/seccomp.txt#L15 I was expecting flags to include SECCOMP_FILTER_FLAG_NEW_LISTENER: seccomp$SECCOMP_SET_MODE_FILTER_LISTENER( op const[SECCOMP_SET_MODE_FILTER], flags flags[seccomp_flags_listener], arg ptr[in, sock_fprog]) fd_seccomp (breaks_returns) For the flags: seccomp_flags_listener = SECCOMP_FILTER_FLAG_NEW_LISTENER, SECCOMP_FILTER_FLAG_LOG_LISTENER, SECCOMP_FILTER_FLAG_SPEC_ALLOW_LISTENER which is: SECCOMP_FILTER_FLAG_LOG_LISTENER = 10 SECCOMP_FILTER_FLAG_NEW_LISTENER = 8 SECCOMP_FILTER_FLAG_SPEC_ALLOW = 4 SECCOMP_FILTER_FLAG_SPEC_ALLOW_LISTENER = 12 How is flags 0 above? (Maybe I don't understand the syzkaller reproducer meaning fully?) > r0 = fork() > ptrace(0x10, r0) 0x10 is PTRACE_ATTACH My best guess is there is some LISTENER refcount state we can get into where all the processes die, but a reference is left alive. -Kees -- Kees Cook
