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Once dst has been cached in socket via sk_setup_caps(), it is illegal to call ip_rt_put() (or dst_release()), since sk_setup_caps() did not change dst refcount. We can still dereference it since we hold socket lock. Caugth by syzbot : BUG: KASAN: use-after-free in atomic_dec_return include/asm-generic/atomic-instrumented.h:198 [inline] BUG: KASAN: use-after-free in dst_release+0x27/0xa0 net/core/dst.c:185 Write of size 4 at addr ffff8801c54dc040 by task syz-executor4/20088 CPU: 1 PID: 20088 Comm: syz-executor4 Not tainted 4.16.0+ #376 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:17 [inline] dump_stack+0x1a7/0x27d lib/dump_stack.c:53 print_address_description+0x73/0x250 mm/kasan/report.c:256 kasan_report_error mm/kasan/report.c:354 [inline] kasan_report+0x23c/0x360 mm/kasan/report.c:412 check_memory_region_inline mm/kasan/kasan.c:260 [inline] check_memory_region+0x137/0x190 mm/kasan/kasan.c:267 kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278 atomic_dec_return include/asm-generic/atomic-instrumented.h:198 [inline] dst_release+0x27/0xa0 net/core/dst.c:185 sk_dst_set include/net/sock.h:1812 [inline] sk_dst_reset include/net/sock.h:1824 [inline] sock_setbindtodevice net/core/sock.c:610 [inline] sock_setsockopt+0x431/0x1b20 net/core/sock.c:707 SYSC_setsockopt net/socket.c:1845 [inline] SyS_setsockopt+0x2ff/0x360 net/socket.c:1828 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287 entry_SYSCALL_64_after_hwframe+0x42/0xb7 RIP: 0033:0x4552d9 RSP: 002b:00007f4878126c68 EFLAGS: 00000246 ORIG_RAX: 0000000000000036 RAX: ffffffffffffffda RBX: 00007f48781276d4 RCX: 00000000004552d9 RDX: 0000000000000019 RSI: 0000000000000001 RDI: 0000000000000013 RBP: 000000000072bea0 R08: 0000000000000010 R09: 0000000000000000 R10: 00000000200010c0 R11: 0000000000000246 R12: 00000000ffffffff R13: 0000000000000526 R14: 00000000006fac30 R15: 0000000000000000 Allocated by task 20088: save_stack+0x43/0xd0 mm/kasan/kasan.c:447 set_track mm/kasan/kasan.c:459 [inline] kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:552 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489 kmem_cache_alloc+0x12e/0x760 mm/slab.c:3542 dst_alloc+0x11f/0x1a0 net/core/dst.c:104 rt_dst_alloc+0xe9/0x540 net/ipv4/route.c:1520 __mkroute_output net/ipv4/route.c:2265 [inline] ip_route_output_key_hash_rcu+0xa49/0x2c60 net/ipv4/route.c:2493 ip_route_output_key_hash+0x20b/0x370 net/ipv4/route.c:2322 __ip_route_output_key include/net/route.h:126 [inline] ip_route_output_flow+0x26/0xa0 net/ipv4/route.c:2577 ip_route_output_ports include/net/route.h:163 [inline] pptp_connect+0xa84/0x1170 drivers/net/ppp/pptp.c:453 SYSC_connect+0x213/0x4a0 net/socket.c:1639 SyS_connect+0x24/0x30 net/socket.c:1620 do_syscall_64+0x281/0x940 arch/x86/entry/common.c:287 entry_SYSCALL_64_after_hwframe+0x42/0xb7 Freed by task 20082: save_stack+0x43/0xd0 mm/kasan/kasan.c:447 set_track mm/kasan/kasan.c:459 [inline] __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:520 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:527 __cache_free mm/slab.c:3486 [inline] kmem_cache_free+0x83/0x2a0 mm/slab.c:3744 dst_destroy+0x266/0x380 net/core/dst.c:140 dst_destroy_rcu+0x16/0x20 net/core/dst.c:153 __rcu_reclaim kernel/rcu/rcu.h:178 [inline] rcu_do_batch kernel/rcu/tree.c:2675 [inline] invoke_rcu_callbacks kernel/rcu/tree.c:2930 [inline] __rcu_process_callbacks kernel/rcu/tree.c:2897 [inline] rcu_process_callbacks+0xd6c/0x17b0 kernel/rcu/tree.c:2914 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285 The buggy address belongs to the object at ffff8801c54dc000 which belongs to the cache ip_dst_cache of size 168 The buggy address is located 64 bytes inside of 168-byte region [ffff8801c54dc000, ffff8801c54dc0a8) The buggy address belongs to the page: page:ffffea0007153700 count:1 mapcount:0 mapping:ffff8801c54dc000 index:0x0 flags: 0x2fffc0000000100(slab) raw: 02fffc0000000100 ffff8801c54dc000 0000000000000000 0000000100000010 raw: ffffea0006b34b20 ffffea0006b6c1e0 ffff8801d674a1c0 0000000000000000 page dumped because: kasan: bad access detected Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
See Documentation/00-INDEX for a list of what is contained in each file.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.