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x86/early_printk: Harden early_serial

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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
This commit is contained in:
Peter Zijlstra
2025-02-07 13:15:38 +01:00
parent c4239a72a2
commit 306859de59
1 changed files with 24 additions and 25 deletions
Download Patch File Download Diff File Expand all files Collapse all files

49
arch/x86/kernel/early_printk.c
View File

@@ -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);