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bpf: enhance validation of pointer formatting #4684

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Pull request for series with
subject: bpf: enhance validation of pointer formatting
version: 3
url: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895

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Upstream branch: adc2186
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: adc2186
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: adc2186
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: fc39fb5
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: fc39fb5
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: fc39fb5
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 28eb75e
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 9f16d5e
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 8618f5f
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 20a39ea
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: ef3ba8c
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 6b64128
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 6b64128
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 3448ad2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 3448ad2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 3448ad2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

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Upstream branch: 3448ad2
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

Fuzzing reports a warning in format_decode()

Please remove unsupported %� in format string
WARNING: CPU: 0 PID: 5091 at lib/vsprintf.c:2680 format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Modules linked in:
CPU: 0 PID: 5091 Comm: syz-executor879 Not tainted 6.10.0-rc1-syzkaller-00021-ge0cce98fe279 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/02/2024
RIP: 0010:format_decode+0x1193/0x1bb0 lib/vsprintf.c:2680
Call Trace:
 <TASK>
 bstr_printf+0x137/0x1210 lib/vsprintf.c:3253
 ____bpf_trace_printk kernel/trace/bpf_trace.c:390 [inline]
 bpf_trace_printk+0x1a1/0x230 kernel/trace/bpf_trace.c:375
 bpf_prog_21da1b68f62e1237+0x36/0x41
 bpf_dispatcher_nop_func include/linux/bpf.h:1243 [inline]
 __bpf_prog_run include/linux/filter.h:691 [inline]
 bpf_prog_run include/linux/filter.h:698 [inline]
 bpf_test_run+0x40b/0x910 net/bpf/test_run.c:425
 bpf_prog_test_run_skb+0xafa/0x13a0 net/bpf/test_run.c:1066
 bpf_prog_test_run+0x33c/0x3b0 kernel/bpf/syscall.c:4291
 __sys_bpf+0x48d/0x810 kernel/bpf/syscall.c:5705
 __do_sys_bpf kernel/bpf/syscall.c:5794 [inline]
 __se_sys_bpf kernel/bpf/syscall.c:5792 [inline]
 __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5792
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The problem occurs when trying to pass %p% at the end of format string,
which would result in skipping last % and passing invalid format string
down to format_decode() that would cause warning because of invalid
character after %.

Fix issue by advancing pointer only if next char is format modifier.
If next char is null/space/punct, then just accept formatting as is,
without advancing the pointer.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Reported-by: syzbot+e2c932aec5c8a6e1d31c@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e2c932aec5c8a6e1d31c
Fixes: 48cac3f ("bpf: Implement formatted output helpers with bstr_printf")
Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>
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Upstream branch: 537a252
series: https://patchwork.kernel.org/project/netdevbpf/list/?series=910895
version: 3

Extend snprintf negative tests to cover pointer specifiers to prevent
possible invalid handling of %p% from happening again.

 ./test_progs -t snprintf
 #302/1   snprintf/snprintf_positive:OK
 #302/2   snprintf/snprintf_negative:OK
 #302     snprintf:OK
 #303     snprintf_btf:OK
 Summary: 2/2 PASSED, 0 SKIPPED, 0 FAILED

Co-developed-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Nikita Marushkin <hfggklm@gmail.com>
Signed-off-by: Ilya Shchipletsov <rabbelkin@mail.ru>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Florent Revest <revest@chromium.org>
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