[PATCH 1/2] target/arm: Clear TTBR[5:0] from a 52-bit table base address

[Fuad Tabba <fuad.tabba@linux.dev>]

get_phys_addr_lpae() can read the first descriptor of a walk from the
wrong address. This happens with a 52-bit OA when the initial lookup
table has fewer than eight entries and its base address has bit 51 set,
or bit 50 for a two-entry table. The walk then faults or translates
through whatever that address holds. VTTBR walks for stage 2 take the
same path.

For example, the 16KB granule with a 48-bit VA has a two-entry level 0
(FEAT_LPA2). The 64KB granule with a 44-bit VA has a four-entry level 1
(FEAT_LPA). KVM's page_fault_test hangs on this in its 16KB, 52-bit PA
guest mode under TCG, as reported on kvmarm.

With a 52-bit OA, TTBR[5:2] hold bits [51:48] of the base address. Such
a table is 64-byte aligned (R_KBLCR), so bits [5:0] of the base address
are zero. get_phys_addr_lpae() ORs TTBR[5:2] into bits [51:48], but
then clears only the bits in indexmask. For these tables indexmask
covers fewer than six bits, so TTBR[5] stays in the address, and
TTBR[4] too for a two-entry table.

Clear TTBR[5:0] when forming a 52-bit table base address, and add a
TCG test that walks a two-entry table through an IPA with bit 50 set,
which stage 2 maps back onto the table.

Reported-by: Itaru Kitayama <itaru.kitayama@linux.dev>
Link: https://lore.kernel.org/kvmarm/B4C0AC3E-6A4F-4A7B-B7BC-81207539115E@linux.dev/
Fixes: 7a928f43d872 ("target/arm: Implement FEAT_LPA")
Cc: qemu-stable@nongnu.org
Signed-off-by: Fuad Tabba <fuad.tabba@linux.dev>
---
 target/arm/ptw.c                      |   5 +-
 tests/tcg/aarch64/system/meson.build  |   6 ++
 tests/tcg/aarch64/system/ttbr-baddr.c | 141 ++++++++++++++++++++++++++
 3 files changed, 151 insertions(+), 1 deletion(-)
 create mode 100644 tests/tcg/aarch64/system/ttbr-baddr.c

diff --git a/target/arm/ptw.c b/target/arm/ptw.c
index de0435a58b..6cff52d592 100644
--- a/target/arm/ptw.c
+++ b/target/arm/ptw.c
@@ -2107,13 +2107,16 @@ static bool get_phys_addr_lpae(CPUARMState *env, S1Translate *ptw,
     descaddr = extract64(ttbr, 0, 48);
 
     /*
-     * For FEAT_LPA and PS=6, bits [51:48] of descaddr are in [5:2] of TTBR.
+     * With a 52-bit OA (FEAT_LPA or FEAT_LPA2), bits [51:48] of descaddr are
+     * in [5:2] of TTBR, and bits [5:0] of the base address are zero: a table
+     * under 64 bytes is still 64-byte aligned (R_KBLCR).
      *
      * Otherwise, if the base address is out of range, raise AddressSizeFault.
      * In the pseudocode, this is !IsZero(baseregister<47:outputsize>),
      * but we've just cleared the bits above 47, so simplify the test.
      */
     if (outputsize > 48) {
+        descaddr &= ~MAKE_64BIT_MASK(0, 6);
         descaddr |= extract64(ttbr, 2, 4) << 48;
     } else if (descaddr >> outputsize) {
         level = 0;
diff --git a/tests/tcg/aarch64/system/meson.build b/tests/tcg/aarch64/system/meson.build
index f51feb253f..270d4d6c31 100644
--- a/tests/tcg/aarch64/system/meson.build
+++ b/tests/tcg/aarch64/system/meson.build
@@ -79,6 +79,12 @@ tests += {
                   '-semihosting-config', 'enable=on,arg=2',
                   qemu_base_args]
   },
+  'ttbr-baddr.c': {
+    'cflags': cflags,
+    'qemu_args': ['-M', 'virt,virtualization=on', '-cpu', 'max',
+                  '-semihosting-config', 'enable=on,arg=2',
+                  qemu_base_args]
+  },
 }
 
 tests += {
diff --git a/tests/tcg/aarch64/system/ttbr-baddr.c b/tests/tcg/aarch64/system/ttbr-baddr.c
new file mode 100644
index 0000000000..7360684268
--- /dev/null
+++ b/tests/tcg/aarch64/system/ttbr-baddr.c
@@ -0,0 +1,141 @@
+/*
+ * TTBR base address with a 52-bit layout and a small initial lookup table
+ *
+ * Copyright (c) 2026 Google LLC
+ * Author: Fuad Tabba <fuad.tabba@linux.dev>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include <stdint.h>
+#include <minilib.h>
+
+/* from Linux's include/linux/stringify.h */
+#define __stringify_1(x...) #x
+#define __stringify(x...)   __stringify_1(x)
+
+#define read_sysreg(r) ({                                           \
+            uint64_t __val;                                         \
+            asm volatile("mrs %0, " __stringify(r) : "=r" (__val)); \
+            __val;                                                  \
+})
+
+#define write_sysreg(r, v) do {                     \
+        uint64_t __val = (uint64_t)(v);             \
+        asm volatile("msr " __stringify(r) ", %x0"  \
+                 : : "rZ" (__val));                 \
+} while (0)
+
+#define RAM_BASE    0x40000000UL
+#define HIGH_IPA    (1UL << 50)
+#define TEST_VA     (0xffff000000000000UL | RAM_BASE)
+
+/*
+ * Stage 1: 16KB granule, 48-bit VA, so level 0 has two entries.
+ * Stage 2: 64KB granule, 52-bit IPA, mapping RAM at its own address and
+ * again at HIGH_IPA.
+ */
+static uint64_t s1_l0[2048] __attribute__((aligned(16384)));
+static uint64_t s1_l1[2048] __attribute__((aligned(16384)));
+static uint64_t s1_l2[2048] __attribute__((aligned(16384)));
+static uint64_t s2_l1[1024] __attribute__((aligned(65536)));
+static uint64_t s2_l2[8192] __attribute__((aligned(65536)));
+static uint64_t s2_l2_high[8192] __attribute__((aligned(65536)));
+
+#define TCR_T0SZ(x)     ((uint64_t)(x) << 0)
+#define TCR_T1SZ(x)     ((uint64_t)(x) << 16)
+#define TCR_TG0_16K     (2UL << 14)
+#define TCR_TG1_16K     (1UL << 30)
+#define TCR_IPS_52      (6UL << 32)
+#define TCR_DS          (1UL << 59)
+
+#define VTCR_T0SZ(x)    ((uint64_t)(x) << 0)
+#define VTCR_SL0_L1     (2UL << 6)
+#define VTCR_TG0_64K    (1UL << 14)
+#define VTCR_PS_52      (6UL << 16)
+#define VTCR_RES1       (1UL << 31)
+
+#define HCR_VM          (1UL << 0)
+#define HCR_RW          (1UL << 31)
+
+#define PAR_F           (1UL << 0)
+#define PAR_PA(par)     ((par) & 0xfffffffff000UL)
+#define S2_BLOCK        ((1 << 10) | (3 << 6) | (0xf << 2) | 1)
+
+static void tlb_flush(void)
+{
+    asm volatile("dsb sy; tlbi alle1; dsb sy; isb" : : : "memory");
+}
+
+static uint64_t at(uint64_t ttbr1)
+{
+    uint64_t par;
+
+    write_sysreg(ttbr1_el1, ttbr1);
+    tlb_flush();
+    asm volatile("at s12e1r, %1; isb; mrs %0, par_el1"
+                 : "=r" (par) : "r" (TEST_VA));
+    return par;
+}
+
+static void setup_tables(void)
+{
+    /* Stage 1: TEST_VA -> RAM_BASE, 32MB block, AF */
+    s1_l0[0] = (uint64_t)s1_l1 | 3;
+    s1_l1[0] = (uint64_t)s1_l2 | 3;
+    s1_l2[(RAM_BASE >> 25) & 0x7ff] = RAM_BASE | (1 << 10) | 1;
+
+    /* Stage 2: 512MB blocks at RAM_BASE and HIGH_IPA | RAM_BASE, AF, RW */
+    s2_l1[0] = (uint64_t)s2_l2 | 3;
+    s2_l1[HIGH_IPA >> 42] = (uint64_t)s2_l2_high | 3;
+    s2_l2[RAM_BASE >> 29] = RAM_BASE | S2_BLOCK;
+    s2_l2_high[RAM_BASE >> 29] = RAM_BASE | S2_BLOCK;
+}
+
+static int check(const char *name, uint64_t par)
+{
+    int ok = !(par & PAR_F) && PAR_PA(par) == RAM_BASE;
+
+    ml_printf("%s: PAR_EL1=%lx %s\n", name, par, ok ? "ok" : "FAIL");
+    return !ok;
+}
+
+int main(void)
+{
+    uint64_t mmfr0 = read_sysreg(id_aa64mmfr0_el1);
+    uint64_t tcr = TCR_T0SZ(16) | TCR_T1SZ(16) | TCR_TG0_16K | TCR_TG1_16K |
+                   TCR_DS;
+    uint64_t base = (uint64_t)s1_l0;
+    int ret = 0;
+
+    ml_printf("TTBR base address test\n");
+
+    /* PARange 52 bits, TGran16 with 52-bit addresses (FEAT_LPA2) */
+    if ((mmfr0 & 0xf) != 6 || ((mmfr0 >> 20) & 0xf) != 2) {
+        ml_printf("SKIP: no 52-bit PA or no FEAT_LPA2 with 16KB\n");
+        return 0;
+    }
+
+    /*
+     * The test runs at EL2 (arg=2) and walks the EL1&0 regime with AT, so
+     * the EL1 MMU settings below only affect those walks.
+     */
+    setup_tables();
+    write_sysreg(mair_el1, 0xff);
+    write_sysreg(sctlr_el1, read_sysreg(sctlr_el1) | 1);
+
+    /*
+     * 52-bit OA: TTBR[4] is base address bit 50. Through stage 2, the
+     * table at HIGH_IPA | s1_l0 is s1_l0, so the walk succeeds only if
+     * TTBR[4] does not also offset the two-entry table by 0x10.
+     */
+    write_sysreg(vtcr_el2, VTCR_T0SZ(12) | VTCR_SL0_L1 | VTCR_TG0_64K |
+                 VTCR_PS_52 | VTCR_RES1);
+    write_sysreg(vttbr_el2, (uint64_t)s2_l1);
+    write_sysreg(hcr_el2, HCR_RW | HCR_VM);
+    write_sysreg(tcr_el1, tcr | TCR_IPS_52);
+    ret |= check("ips52", at(base));
+    ret |= check("ips52 ttbr[4]", at(base | (1 << 4)));
+
+    return ret;
+}
-- 
2.39.5