From a5f7a5bb3b7f28ba7e4fa246775b29a0e5537255 Mon Sep 17 00:00:00 2001 From: Jinke Han Date: Tue, 8 Sep 2026 15:37:42 +0800 Subject: x86/kprobes: Fix crash when probing CS CALL instructions When using eBPF to probe CS CALL instructions within a function, a crash can be triggered. The eBPF tool probes offset 257 of the __hrtimer_run_queues() function: <__hrtimer_run_queues+249>: nopl 0x0(%rax,%rax,1) <__hrtimer_run_queues+254>: mov %r14,%rdi <__hrtimer_run_queues+257>: cs call <__x86_indirect_thunk_r12> <__hrtimer_run_queues+263>: mov %eax,%r12d <__hrtimer_run_queues+266>: xchg %ax,%ax <__hrtimer_run_queues+268>: mov %r13,%rdi Which triggers this crash: BUG: unable to handle page fault for address: 00000000000f41c9 #PF: supervisor write access in kernel mode #PF: error_code(0x0002) - not-present page PGD 0 P4D 0 Oops: 0002 [#1] SMP NOPTI CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P RIP: 0010:__hrtimer_run_queues+0x106/0x230 Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is at the 6th byte of the above CS CALL instruction. Since the CS CALL instruction occupies 6 bytes, the exception occurred in the middle of that call instruction. The root cause is that when using eBPF tools to probe in the middle of a function, a kprobe with INT3 is used as the underlying implementation. During single-step emulation of the original CALL instruction, int3_emulate_call() assumes that the probed CALL instruction is 5 bytes long. However, the actual CS-prefixed CALL instruction occupies 6 bytes, so it constructs an incorrect exception return address. When the CPU returns from the kprobe handler, the next instruction to be executed is at the address of the last byte of that CS CALL instruction. Coincidentally, starting from that address, the CPU fetches and decodes a completely different instruction, which ultimately triggers a kernel crash. Fix the issue by using the actual instruction length obtained from the instruction decoder when constructing the exception return address, rather than relying on the hardcoded CALL_INSN_SIZE macro. [ mingo: Refined the changelog ] Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step") Suggested-by: Masami Hiramatsu (Google) Signed-off-by: Jinke Han Signed-off-by: Ingo Molnar Reviewed-by: Masami Hiramatsu (Google) Acked-by: Yafang Shao Acked-by: Borislav Petkov Cc: Peter Zijlstra Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000 --- arch/alpha/lib/ev6-divide.S | 263 ++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 263 insertions(+) create mode 100644 arch/alpha/lib/ev6-divide.S (limited to 'arch/alpha/lib/ev6-divide.S') diff --git a/arch/alpha/lib/ev6-divide.S b/arch/alpha/lib/ev6-divide.S new file mode 100644 index 000000000..b73a6d263 --- /dev/null +++ b/arch/alpha/lib/ev6-divide.S @@ -0,0 +1,263 @@ +/* SPDX-License-Identifier: GPL-2.0 */ +/* + * arch/alpha/lib/ev6-divide.S + * + * 21264 version contributed by Rick Gorton + * + * Alpha division.. + */ + +/* + * The alpha chip doesn't provide hardware division, so we have to do it + * by hand. The compiler expects the functions + * + * __divqu: 64-bit unsigned long divide + * __remqu: 64-bit unsigned long remainder + * __divqs/__remqs: signed 64-bit + * __divlu/__remlu: unsigned 32-bit + * __divls/__remls: signed 32-bit + * + * These are not normal C functions: instead of the normal + * calling sequence, these expect their arguments in registers + * $24 and $25, and return the result in $27. Register $28 may + * be clobbered (assembly temporary), anything else must be saved. + * + * In short: painful. + * + * This is a rather simple bit-at-a-time algorithm: it's very good + * at dividing random 64-bit numbers, but the more usual case where + * the divisor is small is handled better by the DEC algorithm + * using lookup tables. This uses much less memory, though, and is + * nicer on the cache.. Besides, I don't know the copyright status + * of the DEC code. + */ + +/* + * My temporaries: + * $0 - current bit + * $1 - shifted divisor + * $2 - modulus/quotient + * + * $23 - return address + * $24 - dividend + * $25 - divisor + * + * $27 - quotient/modulus + * $28 - compare status + * + * Much of the information about 21264 scheduling/coding comes from: + * Compiler Writer's Guide for the Alpha 21264 + * abbreviated as 'CWG' in other comments here + * ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html + * Scheduling notation: + * E - either cluster + * U - upper subcluster; U0 - subcluster U0; U1 - subcluster U1 + * L - lower subcluster; L0 - subcluster L0; L1 - subcluster L1 + * Try not to change the actual algorithm if possible for consistency. + */ + +#include +#define halt .long 0 + +/* + * Select function type and registers + */ +#define mask $0 +#define divisor $1 +#define compare $28 +#define tmp1 $3 +#define tmp2 $4 + +#ifdef DIV +#define DIV_ONLY(x,y...) x,##y +#define MOD_ONLY(x,y...) +#define func(x) __div##x +#define modulus $2 +#define quotient $27 +#define GETSIGN(x) xor $24,$25,x +#define STACK 48 +#else +#define DIV_ONLY(x,y...) +#define MOD_ONLY(x,y...) x,##y +#define func(x) __rem##x +#define modulus $27 +#define quotient $2 +#define GETSIGN(x) bis $24,$24,x +#define STACK 32 +#endif + +/* + * For 32-bit operations, we need to extend to 64-bit + */ +#ifdef INTSIZE +#define ufunction func(lu) +#define sfunction func(l) +#define LONGIFY(x) zapnot x,15,x +#define SLONGIFY(x) addl x,0,x +#else +#define ufunction func(qu) +#define sfunction func(q) +#define LONGIFY(x) +#define SLONGIFY(x) +#endif + +.set noat +.align 4 +.globl ufunction +.ent ufunction +ufunction: + subq $30,STACK,$30 # E : + .frame $30,STACK,$23 + .prologue 0 + +7: stq $1, 0($30) # L : + bis $25,$25,divisor # E : + stq $2, 8($30) # L : L U L U + + bis $24,$24,modulus # E : + stq $0,16($30) # L : + bis $31,$31,quotient # E : + LONGIFY(divisor) # E : U L L U + + stq tmp1,24($30) # L : + LONGIFY(modulus) # E : + bis $31,1,mask # E : + DIV_ONLY(stq tmp2,32($30)) # L : L U U L + + beq divisor, 9f /* div by zero */ + /* + * In spite of the DIV_ONLY being either a non-instruction + * or an actual stq, the addition of the .align directive + * below ensures that label 1 is going to be nicely aligned + */ + + .align 4 +#ifdef INTSIZE + /* + * shift divisor left, using 3-bit shifts for + * 32-bit divides as we can't overflow. Three-bit + * shifts will result in looping three times less + * here, but can result in two loops more later. + * Thus using a large shift isn't worth it (and + * s8add pairs better than a sll..) + */ +1: cmpult divisor,modulus,compare # E : + s8addq divisor,$31,divisor # E : + s8addq mask,$31,mask # E : + bne compare,1b # U : U L U L +#else +1: cmpult divisor,modulus,compare # E : + nop # E : + nop # E : + blt divisor, 2f # U : U L U L + + addq divisor,divisor,divisor # E : + addq mask,mask,mask # E : + unop # E : + bne compare,1b # U : U L U L +#endif + + /* ok, start to go right again.. */ +2: + /* + * Keep things nicely bundled... use a nop instead of not + * having an instruction for DIV_ONLY + */ +#ifdef DIV + DIV_ONLY(addq quotient,mask,tmp2) # E : +#else + nop # E : +#endif + srl mask,1,mask # U : + cmpule divisor,modulus,compare # E : + subq modulus,divisor,tmp1 # E : + +#ifdef DIV + DIV_ONLY(cmovne compare,tmp2,quotient) # E : Latency 2, extra map slot + nop # E : as part of the cmovne + srl divisor,1,divisor # U : + nop # E : L U L U + + nop # E : + cmovne compare,tmp1,modulus # E : Latency 2, extra map slot + nop # E : as part of the cmovne + bne mask,2b # U : U L U L +#else + srl divisor,1,divisor # U : + cmovne compare,tmp1,modulus # E : Latency 2, extra map slot + nop # E : as part of the cmovne + bne mask,2b # U : U L L U +#endif + +9: ldq $1, 0($30) # L : + ldq $2, 8($30) # L : + nop # E : + nop # E : U U L L + + ldq $0,16($30) # L : + ldq tmp1,24($30) # L : + nop # E : + nop # E : + +#ifdef DIV + DIV_ONLY(ldq tmp2,32($30)) # L : +#else + nop # E : +#endif + addq $30,STACK,$30 # E : + ret $31,($23),1 # L0 : L U U L + .end ufunction +EXPORT_SYMBOL(ufunction) + +/* + * Uhh.. Ugly signed division. I'd rather not have it at all, but + * it's needed in some circumstances. There are different ways to + * handle this, really. This does: + * -a / b = a / -b = -(a / b) + * -a % b = -(a % b) + * a % -b = a % b + * which is probably not the best solution, but at least should + * have the property that (x/y)*y + (x%y) = x. + */ +.align 4 +.globl sfunction +.ent sfunction +sfunction: + subq $30,STACK,$30 # E : + .frame $30,STACK,$23 + .prologue 0 + bis $24,$25,$28 # E : + SLONGIFY($28) # E : + bge $28,7b # U : + + stq $24,0($30) # L : + subq $31,$24,$28 # E : + stq $25,8($30) # L : + nop # E : U L U L + + cmovlt $24,$28,$24 /* abs($24) */ # E : Latency 2, extra map slot + nop # E : as part of the cmov + stq $23,16($30) # L : + subq $31,$25,$28 # E : U L U L + + stq tmp1,24($30) # L : + cmovlt $25,$28,$25 /* abs($25) */ # E : Latency 2, extra map slot + nop # E : + bsr $23,ufunction # L0: L U L U + + ldq $24,0($30) # L : + ldq $25,8($30) # L : + GETSIGN($28) # E : + subq $31,$27,tmp1 # E : U U L L + + SLONGIFY($28) # E : + ldq $23,16($30) # L : + cmovlt $28,tmp1,$27 # E : Latency 2, extra map slot + nop # E : U L L U : as part of the cmov + + ldq tmp1,24($30) # L : + nop # E : as part of the cmov + addq $30,STACK,$30 # E : + ret $31,($23),1 # L0 : L U U L + .end sfunction +EXPORT_SYMBOL(sfunction) -- cgit v1.3.1