CVE-2022-48998: In the Linux kernel, the following vulnerability has been resolved:  powerpc/bpf/32: Fix Oops on tail call tests  test_bpf tail call tests end up as:    test_bpf: #0 Tail call leaf jited:1 85 PASS   test_bpf: #1 Tail call 2 jited:1 111 PASS   test_bpf: #2 Tail call 3 jited:1 145 PASS   test_bpf: #3 Tail call 4 jited:1 170 PASS   test_bpf: #4 Tail call load/store leaf jited:1 190 PASS   test_bpf: #5 Tail call load/store jited:1   BUG: Unable to handle kernel data access on write at 0xf1b4e000   Faulting instruction address: 0xbe86b710   Oops: Kernel access of bad area, sig: 11 [#1]   BE PAGE_SIZE=4K MMU=Hash PowerMac   Modules linked in: test_bpf(+)   CPU: 0 PID: 97 Comm: insmod Not tainted 6.1.0-rc4+ #195   Hardware name: PowerMac3,1 750CL 0x87210 PowerMac   NIP:  be86b710 LR: be857e88 CTR: be86b704   REGS: f1b4df20 TRAP: 0300   Not tainted  (6.1.0-rc4+)   MSR:  00009032 <EE,ME,IR,DR,RI>  CR: 28008242  XER: 00000000   DAR: f1b4e000 DSISR: 42000000   GPR00: 00000001 f1b4dfe0 c11d2280 00000000 00000000 00000000 00000002 00000000   GPR08: f1b4e000 be86b704 f1b4e000 00000000 00000000 100d816a f2440000 fe73baa8   GPR16: f2458000 00000000 c1941ae4 f1fe2248 00000045 c0de0000 f2458030 00000000   GPR24: 000003e8 0000000f f2458000 f1b4dc90 3e584b46 00000000 f24466a0 c1941a00   NIP [be86b710] 0xbe86b710   LR [be857e88] __run_one+0xec/0x264 [test_bpf]   Call Trace:   [f1b4dfe0] [00000002] 0x2 (unreliable)   Instruction dump:   XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX   XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX   ---[ end trace 0000000000000000 ]---  This is a tentative to write above the stack. The problem is encoutered with tests added by commit 38608ee7b690 ("bpf, tests: Add load store test case for tail call")  This happens because tail call is done to a BPF prog with a different stack_depth. At the time being, the stack is kept as is when the caller tail calls its callee. But at exit, the callee restores the stack based on its own properties. Therefore here, at each run, r1 is erroneously increased by 32 - 16 = 16 bytes.  This was done that way in order to pass the tail call count from caller to callee through the stack. As powerpc32 doesn't have a red zone in the stack, it was necessary the maintain the stack as is for the tail call. But it was not anticipated that the BPF frame size could be different.  Let's take a new approach. Use register r4 to carry the tail call count during the tail call, and save it into the stack at function entry if required. This means the input parameter must be in r3, which is more correct as it is a 32 bits parameter, then tail call better match with normal BPF function entry, the down side being that we move that input parameter back and forth between r3 and r4. That can be optimised later.  Doing that also has the advantage of maximising the common parts between tail calls and a normal function exit.  With the fix, tail call tests are now successfull:    test_bpf: #0 Tail call leaf jited:1 53 PASS   test_bpf: #1 Tail call 2 jited:1 115 PASS   test_bpf: #2 Tail call 3 jited:1 154 PASS   test_bpf: #3 Tail call 4 jited:1 165 PASS   test_bpf: #4 Tail call load/store leaf jited:1 101 PASS   test_bpf: #5 Tail call load/store jited:1 141 PASS   test_bpf: #6 Tail call error path, max count reached jited:1 994 PASS   test_bpf: #7 Tail call count preserved across function calls jited:1 140975 PASS   test_bpf: #8 Tail call error path, NULL target jited:1 110 PASS   test_bpf: #9 Tail call error path, index out of range jited:1 69 PASS   test_bpf: test_tail_calls: Summary: 10 PASSED, 0 FAILED, [10/10 JIT'ed]