x86/early_printk: Harden early_serial

Scott found that mem32_serial_in() is an ideal speculation gadget, an
indirectly callable function that takes an adddress and offset and
immediately does a load.

Use static_call() to take away the need for indirect calls and
explicitly seal the functions to ensure they're not callable on IBT
enabled parts.

Reported-by: Scott Constable <scott.d.constable@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.919773202@infradead.org

diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index 44f937015e1e25bf41532eb7e1031a6be32a6523..fc1714bad04588abc3468a0e72df26513250162b 100644 (file)
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -19,6 +19,7 @@
 #include <linux/usb/ehci_def.h>
 #include <linux/usb/xhci-dbgp.h>
 #include <asm/pci_x86.h>
+#include <linux/static_call.h>
 
 /* Simple VGA output */
 #define VGABASE                (__ISA_IO_base + 0xb8000)
@@ -94,26 +95,28 @@ static unsigned long early_serial_base = 0x3f8;  /* ttyS0 */
 #define DLL             0       /*  Divisor Latch Low         */
 #define DLH             1       /*  Divisor latch High        */
 
-static unsigned int io_serial_in(unsigned long addr, int offset)
+static __noendbr unsigned int io_serial_in(unsigned long addr, int offset)
 {
        return inb(addr + offset);
 }
+ANNOTATE_NOENDBR_SYM(io_serial_in);
 
-static void io_serial_out(unsigned long addr, int offset, int value)
+static __noendbr void io_serial_out(unsigned long addr, int offset, int value)
 {
        outb(value, addr + offset);
 }
+ANNOTATE_NOENDBR_SYM(io_serial_out);
 
-static unsigned int (*serial_in)(unsigned long addr, int offset) = io_serial_in;
-static void (*serial_out)(unsigned long addr, int offset, int value) = io_serial_out;
+DEFINE_STATIC_CALL(serial_in, io_serial_in);
+DEFINE_STATIC_CALL(serial_out, io_serial_out);
 
 static int early_serial_putc(unsigned char ch)
 {
        unsigned timeout = 0xffff;
 
-       while ((serial_in(early_serial_base, LSR) & XMTRDY) == 0 && --timeout)
+       while ((static_call(serial_in)(early_serial_base, LSR) & XMTRDY) == 0 && --timeout)
                cpu_relax();
-       serial_out(early_serial_base, TXR, ch);
+       static_call(serial_out)(early_serial_base, TXR, ch);
        return timeout ? 0 : -1;
 }
 
@@ -131,16 +134,16 @@ static __init void early_serial_hw_init(unsigned divisor)
 {
        unsigned char c;
 
-       serial_out(early_serial_base, LCR, 0x3);        /* 8n1 */
-       serial_out(early_serial_base, IER, 0);  /* no interrupt */
-       serial_out(early_serial_base, FCR, 0);  /* no fifo */
-       serial_out(early_serial_base, MCR, 0x3);        /* DTR + RTS */
+       static_call(serial_out)(early_serial_base, LCR, 0x3);   /* 8n1 */
+       static_call(serial_out)(early_serial_base, IER, 0);     /* no interrupt */
+       static_call(serial_out)(early_serial_base, FCR, 0);     /* no fifo */
+       static_call(serial_out)(early_serial_base, MCR, 0x3);   /* DTR + RTS */
 
-       c = serial_in(early_serial_base, LCR);
-       serial_out(early_serial_base, LCR, c | DLAB);
-       serial_out(early_serial_base, DLL, divisor & 0xff);
-       serial_out(early_serial_base, DLH, (divisor >> 8) & 0xff);
-       serial_out(early_serial_base, LCR, c & ~DLAB);
+       c = static_call(serial_in)(early_serial_base, LCR);
+       static_call(serial_out)(early_serial_base, LCR, c | DLAB);
+       static_call(serial_out)(early_serial_base, DLL, divisor & 0xff);
+       static_call(serial_out)(early_serial_base, DLH, (divisor >> 8) & 0xff);
+       static_call(serial_out)(early_serial_base, LCR, c & ~DLAB);
 }
 
 #define DEFAULT_BAUD 9600
@@ -183,28 +186,26 @@ static __init void early_serial_init(char *s)
        /* Convert from baud to divisor value */
        divisor = 115200 / baud;
 
-       /* These will always be IO based ports */
-       serial_in = io_serial_in;
-       serial_out = io_serial_out;
-
        /* Set up the HW */
        early_serial_hw_init(divisor);
 }
 
 #ifdef CONFIG_PCI
-static void mem32_serial_out(unsigned long addr, int offset, int value)
+static __noendbr void mem32_serial_out(unsigned long addr, int offset, int value)
 {
        u32 __iomem *vaddr = (u32 __iomem *)addr;
        /* shift implied by pointer type */
        writel(value, vaddr + offset);
 }
+ANNOTATE_NOENDBR_SYM(mem32_serial_out);
 
-static unsigned int mem32_serial_in(unsigned long addr, int offset)
+static __noendbr unsigned int mem32_serial_in(unsigned long addr, int offset)
 {
        u32 __iomem *vaddr = (u32 __iomem *)addr;
        /* shift implied by pointer type */
        return readl(vaddr + offset);
 }
+ANNOTATE_NOENDBR_SYM(mem32_serial_in);
 
 /*
  * early_pci_serial_init()
@@ -278,15 +279,13 @@ static __init void early_pci_serial_init(char *s)
         */
        if ((bar0 & PCI_BASE_ADDRESS_SPACE) == PCI_BASE_ADDRESS_SPACE_IO) {
                /* it is IO mapped */
-               serial_in = io_serial_in;
-               serial_out = io_serial_out;
                early_serial_base = bar0 & PCI_BASE_ADDRESS_IO_MASK;
                write_pci_config(bus, slot, func, PCI_COMMAND,
                                 cmdreg|PCI_COMMAND_IO);
        } else {
                /* It is memory mapped - assume 32-bit alignment */
-               serial_in = mem32_serial_in;
-               serial_out = mem32_serial_out;
+               static_call_update(serial_in, mem32_serial_in);
+               static_call_update(serial_out, mem32_serial_out);
                /* WARNING! assuming the address is always in the first 4G */
                early_serial_base =
                        (unsigned long)early_ioremap(bar0 & PCI_BASE_ADDRESS_MEM_MASK, 0x10);