[PATCH v2] RISC-V: KVM: Fix PMU event info array size overflow

[Guidong Han]

SBI PMU EVENT_GET_INFO stores guest-controlled
num_events * sizeof(*einfo) in an int. On RV64, num_events = 0x10000001
makes 0x100000010 truncate to 16. KVM then allocates one entry but loops
over the original num_events, causing out-of-bounds reads and writes. A
nested guest triggered:

BUG: KASAN: slab-out-of-bounds in kvm_riscv_vcpu_pmu_event_info+0xa4/0x142
Read of size 4 at addr ff600000074d46b0 by task init/1
Call Trace:
[<ffffffff8006471c>] kvm_riscv_vcpu_pmu_event_info+0xa4/0x142
[<ffffffff800690c0>] kvm_sbi_ext_pmu_handler+0xca/0x268
[<ffffffff8006779e>] kvm_riscv_vcpu_sbi_ecall+0xec/0x1e6
[<ffffffff8006008c>] kvm_riscv_vcpu_exit+0x48c/0x540
[<ffffffff8005ea0a>] kvm_arch_vcpu_ioctl_run+0x37e/0xc80
Allocated by task 1:
 __kmalloc_noprof+0x19e/0x4b0
 kvm_riscv_vcpu_pmu_event_info+0x72/0x142
 kvm_sbi_ext_pmu_handler+0xca/0x268
 kvm_riscv_vcpu_sbi_ecall+0xec/0x1e6
 kvm_riscv_vcpu_exit+0x48c/0x540
 kvm_arch_vcpu_ioctl_run+0x37e/0xc80
The buggy address is located 0 bytes to the right of
 allocated 16-byte region [ff600000074d46a0, ff600000074d46b0)

Store the shared-memory size in size_t and reject multiplication overflow.
Allocate the guest-driven array with GFP_KERNEL_ACCOUNT so it is charged
to kmemcg, and use __GFP_NOWARN to suppress allocation failure warnings.
Use kvcalloc() to allow vmalloc fallback and an unsigned long loop index
to match num_events.

Reported-by: Naveed Khan <naveed at digiscrypt.com>
Closes: https://lore.kernel.org/kvm/178345245327.72065.13249716450708539854@digiscrypt.com/
Fixes: e309fd113b9f ("RISC-V: KVM: Implement get event info function")
Cc: stable at vger.kernel.org
Signed-off-by: Guidong Han <2045gemini at gmail.com>
---
Changes in v2:
- Use an accounted, no-warning kvcalloc() allocation and kvfree(), as
  suggested by Sashiko.
- Naveed Khan reported the issue before v1. I missed his report at the
  time, so add Reported-by and Closes tags to credit him.

Link to v1: https://lore.kernel.org/kvm/20260718153620.829378-1-2045gemini@gmail.com/
---
 arch/riscv/kvm/vcpu_pmu.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
index bb46dcbfb24d..8e1f6abb84e2 100644
--- a/arch/riscv/kvm/vcpu_pmu.c
+++ b/arch/riscv/kvm/vcpu_pmu.c
@@ -12,7 +12,9 @@
 #include <linux/err.h>
 #include <linux/kvm_host.h>
 #include <linux/nospec.h>
+#include <linux/overflow.h>
 #include <linux/perf/riscv_pmu.h>
+#include <linux/slab.h>
 #include <asm/csr.h>
 #include <asm/kvm_isa.h>
 #include <asm/kvm_vcpu_sbi.h>
@@ -479,13 +481,14 @@ int kvm_riscv_vcpu_pmu_event_info(struct kvm_vcpu *vcpu, unsigned long saddr_low
 				  unsigned long flags, struct kvm_vcpu_sbi_return *retdata)
 {
 	struct riscv_pmu_event_info *einfo = NULL;
-	int shmem_size = num_events * sizeof(*einfo);
+	size_t shmem_size;
 	gpa_t shmem;
 	u32 eidx, etype;
 	u64 econfig;
 	int ret;
 
-	if (flags != 0 || (saddr_low & (SZ_16 - 1) || num_events == 0)) {
+	if (flags != 0 || (saddr_low & (SZ_16 - 1)) || num_events == 0 ||
+	    check_mul_overflow(num_events, sizeof(*einfo), &shmem_size)) {
 		ret = SBI_ERR_INVALID_PARAM;
 		goto out;
 	}
@@ -500,7 +503,8 @@ int kvm_riscv_vcpu_pmu_event_info(struct kvm_vcpu *vcpu, unsigned long saddr_low
 		}
 	}
 
-	einfo = kzalloc(shmem_size, GFP_KERNEL);
+	einfo = kvcalloc(num_events, sizeof(*einfo),
+			 GFP_KERNEL_ACCOUNT | __GFP_NOWARN);
 	if (!einfo) {
 		ret = SBI_ERR_FAILURE;
 		goto out;
@@ -512,7 +516,7 @@ int kvm_riscv_vcpu_pmu_event_info(struct kvm_vcpu *vcpu, unsigned long saddr_low
 		goto free_mem;
 	}
 
-	for (int i = 0; i < num_events; i++) {
+	for (unsigned long i = 0; i < num_events; i++) {
 		eidx = einfo[i].event_idx;
 		etype = kvm_pmu_get_perf_event_type(eidx);
 		econfig = kvm_pmu_get_perf_event_config(eidx, einfo[i].event_data);
@@ -525,7 +529,7 @@ int kvm_riscv_vcpu_pmu_event_info(struct kvm_vcpu *vcpu, unsigned long saddr_low
 		ret = SBI_ERR_INVALID_ADDRESS;
 
 free_mem:
-	kfree(einfo);
+	kvfree(einfo);
 out:
 	retdata->err_val = ret;
 
-- 
2.43.0