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[rng] Restore state of IRQ 8 and PIE when disabling entropy gathering 

Legacy IRQ 8 appears to be enabled by default on some platforms.  If
iPXE selects the RTC entropy source, this will currently result in the
RTC IRQ 8 being unconditionally disabled.  This can break assumptions
made by BIOSes or subsequent bootloaders: in particular, the FreeBSD
loader may lock up at the point of starting its default 10-second
countdown when it calls INT 15,86.

Fix by restoring the previous state of IRQ 8 instead of disabling it
unconditionally.  Note that we do not need to disable IRQ 8 around the
point of hooking (or unhooking) the ISR, since this code will be
executing in iPXE's normal state of having interrupts disabled anyway.

Also restore the previous state of the RTC periodic interrupt enable,
rather than disabling it unconditionally.

Signed-off-by: Michael Brown <mcb30@ipxe.org>
pull/1165/merge
Michael Brown 2025-03-14 14:11:48 +00:00
parent 8840de4096
commit 829e2d1f29
1 changed files with 25 additions and 8 deletions
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33
src/arch/x86/interface/pcbios/rtc_entropy.c
View File

@ -53,6 +53,12 @@ extern void rtc_isr ( void );
/** Previous RTC interrupt handler */
static struct segoff rtc_old_handler;
/** Previous RTC interrupt enabled state */
static uint8_t rtc_irq_enabled;
/** Previous RTC periodic interrupt enabled state */
static uint8_t rtc_int_enabled;
/** Flag set by RTC interrupt handler */
extern volatile uint8_t __text16 ( rtc_flag );
#define rtc_flag __use_text16 ( rtc_flag )
@ -107,8 +113,9 @@ static void rtc_unhook_isr ( void ) {
/**
* Enable RTC interrupts
*
* @ret enabled Periodic interrupt was previously enabled
*/
static void rtc_enable_int ( void ) {
static int rtc_enable_int ( void ) {
uint8_t status_b;
/* Clear any stale pending interrupts via status register C */
@ -124,6 +131,9 @@ static void rtc_enable_int ( void ) {
/* Re-enable NMI and reset to default address */
outb ( CMOS_DEFAULT_ADDRESS, CMOS_ADDRESS );
inb ( CMOS_DATA ); /* Discard; may be needed on some platforms */
/* Return previous state */
return ( status_b & RTC_STATUS_B_PIE );
}
/**
@ -198,8 +208,11 @@ static int rtc_entropy_enable ( void ) {
/* Hook ISR and enable RTC interrupts */
rtc_hook_isr();
enable_irq ( RTC_IRQ );
rtc_enable_int();
rtc_irq_enabled = enable_irq ( RTC_IRQ );
rtc_int_enabled = rtc_enable_int();
DBGC ( &rtc_flag, "RTC had IRQ%d %sabled, interrupt %sabled\n",
RTC_IRQ, ( rtc_irq_enabled ? "en" : "dis" ),
( rtc_int_enabled ? "en" : "dis" ) );
/* Check that RTC interrupts are working */
if ( ( rc = rtc_entropy_check() ) != 0 )
@ -223,8 +236,10 @@ static int rtc_entropy_enable ( void ) {
return 0;
err_check:
rtc_disable_int();
disable_irq ( RTC_IRQ );
if ( ! rtc_int_enabled )
rtc_disable_int();
if ( ! rtc_irq_enabled )
disable_irq ( RTC_IRQ );
rtc_unhook_isr();
err_no_tsc:
return rc;
@ -236,9 +251,11 @@ static int rtc_entropy_enable ( void ) {
*/
static void rtc_entropy_disable ( void ) {
/* Disable RTC interrupts and unhook ISR */
rtc_disable_int();
disable_irq ( RTC_IRQ );
/* Restore RTC interrupt state and unhook ISR */
if ( ! rtc_int_enabled )
rtc_disable_int();
if ( ! rtc_irq_enabled )
disable_irq ( RTC_IRQ );
rtc_unhook_isr();
}