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x86/alternative: Add indirect call patching

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In order to prepare replacing of paravirt patching with alternative
patching, add the capability to replace an indirect call with a direct
one.

This is done via a new flag ALT_FLAG_CALL as the target of the CALL
instruction needs to be evaluated using the value of the location
addressed by the indirect call.

For convenience, add a macro for a default CALL instruction. In case it
is being used without the new flag being set, it will result in a BUG()
when being executed. As in most cases, the feature used will be
X86_FEATURE_ALWAYS so add another macro ALT_CALL_ALWAYS usable for the
flags parameter of the ALTERNATIVE macros.

For a complete replacement, handle the special cases of calling a nop
function and an indirect call of NULL the same way as paravirt does.

  [ bp: Massage commit message, fixup the debug output and clarify flow
    more. ]

Co-developed-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231210062138.2417-4-jgross@suse.com
This commit is contained in:
Juergen Gross 2023-12-10 07:21:36 +01:00 • committed by Borislav Petkov (AMD)
parent 9824b00c2b
commit da0fe6e68e
2 changed files with 64 additions and 3 deletions
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9
arch/x86/include/asm/alternative.h
View file

@ -10,6 +10,9 @@
#define ALT_FLAG_NOT (1 << 0)
#define ALT_NOT(feature) ((ALT_FLAG_NOT << ALT_FLAGS_SHIFT) | (feature))
#define ALT_FLAG_DIRECT_CALL (1 << 1)
#define ALT_DIRECT_CALL(feature) ((ALT_FLAG_DIRECT_CALL << ALT_FLAGS_SHIFT) | (feature))
#define ALT_CALL_ALWAYS ALT_DIRECT_CALL(X86_FEATURE_ALWAYS)
#ifndef __ASSEMBLY__
@ -150,6 +153,8 @@ static inline int alternatives_text_reserved(void *start, void *end)
}
#endif /* CONFIG_SMP */
#define ALT_CALL_INSTR "call BUG_func"
#define b_replacement(num) "664"#num
#define e_replacement(num) "665"#num
@ -386,6 +391,10 @@ void nop_func(void);
.byte \alt_len
.endm
.macro ALT_CALL_INSTR
call BUG_func
.endm
/*
* Define an alternative between two instructions. If @feature is
* present, early code in apply_alternatives() replaces @oldinstr with

58
arch/x86/kernel/alternative.c
View file

@ -395,6 +395,53 @@ noinstr void BUG_func(void)
}
EXPORT_SYMBOL_GPL(BUG_func);
#define CALL_RIP_REL_OPCODE 0xff
#define CALL_RIP_REL_MODRM 0x15
/*
* Rewrite the "call BUG_func" replacement to point to the target of the
* indirect pv_ops call "call *disp(%ip)".
*/
static int alt_replace_call(u8 *instr, u8 *insn_buff, struct alt_instr *a)
{
void *target, *bug = &BUG_func;
s32 disp;
if (a->replacementlen != 5 || insn_buff[0] != CALL_INSN_OPCODE) {
pr_err("ALT_FLAG_DIRECT_CALL set for a non-call replacement instruction\n");
BUG();
}
if (a->instrlen != 6 ||
instr[0] != CALL_RIP_REL_OPCODE ||
instr[1] != CALL_RIP_REL_MODRM) {
pr_err("ALT_FLAG_DIRECT_CALL set for unrecognized indirect call\n");
BUG();
}
/* Skip CALL_RIP_REL_OPCODE and CALL_RIP_REL_MODRM */
disp = *(s32 *)(instr + 2);
#ifdef CONFIG_X86_64
/* ff 15 00 00 00 00 call *0x0(%rip) */
/* target address is stored at "next instruction + disp". */
target = *(void **)(instr + a->instrlen + disp);
#else
/* ff 15 00 00 00 00 call *0x0 */
/* target address is stored at disp. */
target = *(void **)disp;
#endif
if (!target)
target = bug;
/* (BUG_func - .) + (target - BUG_func) := target - . */
*(s32 *)(insn_buff + 1) += target - bug;
if (target == &nop_func)
return 0;
return 5;
}
/*
* Replace instructions with better alternatives for this CPU type. This runs
* before SMP is initialized to avoid SMP problems with self modifying code.
@ -452,16 +499,21 @@ void __init_or_module noinline apply_alternatives(struct alt_instr *start,
continue;
}
DPRINTK(ALT, "feat: %s%d*32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d)",
(a->flags & ALT_FLAG_NOT) ? "!" : "",
DPRINTK(ALT, "feat: %d32+%d, old: (%pS (%px) len: %d), repl: (%px, len: %d) flags: 0x%x",
a->cpuid >> 5,
a->cpuid & 0x1f,
instr, instr, a->instrlen,
replacement, a->replacementlen);
replacement, a->replacementlen, a->flags);
memcpy(insn_buff, replacement, a->replacementlen);
insn_buff_sz = a->replacementlen;
if (a->flags & ALT_FLAG_DIRECT_CALL) {
insn_buff_sz = alt_replace_call(instr, insn_buff, a);
if (insn_buff_sz < 0)
continue;
}
for (; insn_buff_sz < a->instrlen; insn_buff_sz++)
insn_buff[insn_buff_sz] = 0x90;