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Miscellaneous x86 fixes: 

- Fix an early boot crash in AMD SEV-SNP guests, caused by incorrect
    FSGSBASE init ordering. (Nikunj A Dadhania)
 
  - Remove X86_CR4_FRED from the CR4 pinned bits mask, to fix a race
    window during the bootup of SEV-{ES,SNP} or TDX guests, which
    can crash them if they trigger exceptions in that window.
    (Borislav Petkov)
 
  - Fix early boot failures on SEV-ES/SNP guests, due to incorrect
    early GHCB access. (Nikunj A Dadhania)
 
  - Add clarifying comment to the CRn pinning logic, to avoid
    future confusion & bugs. (Peter Zijlstra)
 
 Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'x86-urgent-2026-03-29' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 fixes from Ingo Molnar:

 - Fix an early boot crash in AMD SEV-SNP guests, caused by incorrect
   FSGSBASE init ordering (Nikunj A Dadhania)

 - Remove X86_CR4_FRED from the CR4 pinned bits mask, to fix a race
   window during the bootup of SEV-{ES,SNP} or TDX guests, which can
   crash them if they trigger exceptions in that window (Borislav
   Petkov)

 - Fix early boot failures on SEV-ES/SNP guests, due to incorrect early
   GHCB access (Nikunj A Dadhania)

 - Add clarifying comment to the CRn pinning logic, to avoid future
   confusion & bugs (Peter Zijlstra)

* tag 'x86-urgent-2026-03-29' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/cpu: Add comment clarifying CRn pinning
  x86/fred: Fix early boot failures on SEV-ES/SNP guests
  x86/cpu: Remove X86_CR4_FRED from the CR4 pinned bits mask
  x86/cpu: Enable FSGSBASE early in cpu_init_exception_handling()
This commit is contained in:
Linus Torvalds 2026-03-29 10:04:37 -07:00
parent 47e3f23f0e a3e93cac25
commit f242ac4a09
3 changed files with 46 additions and 7 deletions
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6
arch/x86/coco/sev/noinstr.c
View File

@ -121,6 +121,9 @@ noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
WARN_ON(!irqs_disabled());
if (!sev_cfg.ghcbs_initialized)
return boot_ghcb;
data = this_cpu_read(runtime_data);
ghcb = &data->ghcb_page;
@ -164,6 +167,9 @@ noinstr void __sev_put_ghcb(struct ghcb_state *state)
WARN_ON(!irqs_disabled());
if (!sev_cfg.ghcbs_initialized)
return;
data = this_cpu_read(runtime_data);
ghcb = &data->ghcb_page;

14
arch/x86/entry/entry_fred.c
View File

@ -177,6 +177,16 @@ static noinstr void fred_extint(struct pt_regs *regs)
}
}
#ifdef CONFIG_AMD_MEM_ENCRYPT
noinstr void exc_vmm_communication(struct pt_regs *regs, unsigned long error_code)
{
if (user_mode(regs))
return user_exc_vmm_communication(regs, error_code);
else
return kernel_exc_vmm_communication(regs, error_code);
}
#endif
static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
{
/* Optimize for #PF. That's the only exception which matters performance wise */
@ -207,6 +217,10 @@ static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
#ifdef CONFIG_X86_CET
case X86_TRAP_CP: return exc_control_protection(regs, error_code);
#endif
#ifdef CONFIG_AMD_MEM_ENCRYPT
case X86_TRAP_VC: return exc_vmm_communication(regs, error_code);
#endif
default: return fred_bad_type(regs, error_code);
}

33
arch/x86/kernel/cpu/common.c
View File

@ -433,7 +433,20 @@ static __always_inline void setup_lass(struct cpuinfo_x86 *c)
/* These bits should not change their value after CPU init is finished. */
static const unsigned long cr4_pinned_mask = X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP |
X86_CR4_FSGSBASE | X86_CR4_CET | X86_CR4_FRED;
X86_CR4_FSGSBASE | X86_CR4_CET;
/*
* The CR pinning protects against ROP on the 'mov %reg, %CRn' instruction(s).
* Since you can ROP directly to these instructions (barring shadow stack),
* any protection must follow immediately and unconditionally after that.
*
* Specifically, the CR[04] write functions below will have the value
* validation controlled by the @cr_pinning static_branch which is
* __ro_after_init, just like the cr4_pinned_bits value.
*
* Once set, an attacker will have to defeat page-tables to get around these
* restrictions. Which is a much bigger ask than 'simple' ROP.
*/
static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
static unsigned long cr4_pinned_bits __ro_after_init;
@ -2050,12 +2063,6 @@ static void identify_cpu(struct cpuinfo_x86 *c)
setup_umip(c);
setup_lass(c);
/* Enable FSGSBASE instructions if available. */
if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
cr4_set_bits(X86_CR4_FSGSBASE);
elf_hwcap2 |= HWCAP2_FSGSBASE;
}
/*
* The vendor-specific functions might have changed features.
* Now we do "generic changes."
@ -2416,6 +2423,18 @@ void cpu_init_exception_handling(bool boot_cpu)
/* GHCB needs to be setup to handle #VC. */
setup_ghcb();
/*
* On CPUs with FSGSBASE support, paranoid_entry() uses
* ALTERNATIVE-patched RDGSBASE/WRGSBASE instructions. Secondary CPUs
* boot after alternatives are patched globally, so early exceptions
* execute patched code that depends on FSGSBASE. Enable the feature
* before any exceptions occur.
*/
if (cpu_feature_enabled(X86_FEATURE_FSGSBASE)) {
cr4_set_bits(X86_CR4_FSGSBASE);
elf_hwcap2 |= HWCAP2_FSGSBASE;
}
if (cpu_feature_enabled(X86_FEATURE_FRED)) {
/* The boot CPU has enabled FRED during early boot */
if (!boot_cpu)